Homogeneous Mixtures Chem Lab Solutions Worksheet

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INTRODUCTION

Solutions are homogeneous mixtures of two or more substances. Homogeneous means that when the substances are mixed no difference in solution appearance can be seen (for example, no cloudiness). There are some common terms that are used to describe solutions. A solute is a substance that is dissolved in another. The solvent is the substance in which the solute is dissolved. The solution is composed of the solute(s) and the solvent. For aqueous solutions, the solvent is water. Solutes are not always solids. Sometimes, two liquids are mixed together to form a solution. When two liquids are able to form a solution, they are called miscible liquids (for example alcohol and water).

Concentration is used to describe the amount of solute dissolved in a particular amount of solvent. A molar solution is a solution whose concentration is expressed terms of Molarity (M), which has units of mol solute per liters of solution. Molarity is the most commonly used concentration unit. It is useful for stoichiometric problems since it allows the number of moles of the solute to be related to the volume of the solution used.

Molarity (M) = π‘šπ‘šπ‘šπ‘šπ‘šπ‘š π‘šπ‘šπ‘œπ‘œ π‘ π‘ π‘šπ‘šπ‘šπ‘šπ‘ π‘ π‘ π‘ π‘ π‘  (π‘šπ‘šπ‘šπ‘šπ‘šπ‘š)

𝐿𝐿𝐿𝐿𝑠𝑠𝑠𝑠𝐿𝐿𝑠𝑠 π‘šπ‘šπ‘œπ‘œ π‘ π‘ π‘šπ‘šπ‘šπ‘šπ‘ π‘ π‘ π‘ πΏπΏπ‘šπ‘šπ‘ π‘  (𝐿𝐿)

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Example 1: How many grams of potassium iodide (KI) should be dissolved to prepare a 1.00 L aqueous solution of potassium iodide (KI)?

1.25 π‘šπ‘šπ‘šπ‘šπ‘šπ‘š 𝐾𝐾𝐾𝐾 166.00 𝑔𝑔 𝐾𝐾𝐾𝐾

1.00 𝐿𝐿 π‘₯π‘₯ π‘₯π‘₯ = 𝟐𝟐𝟐𝟐𝟐𝟐 π’ˆπ’ˆ 𝑲𝑲𝑲𝑲

1 𝐿𝐿 1 π‘šπ‘šπ‘šπ‘šπ‘šπ‘š 𝐾𝐾𝐾𝐾

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Example 2: A student uses a 25.0 mL of a 1.5 M solution of NaOH(aq) for a titration experiment. How many moles of NaOH are used in the experiment?

word image 854 1.5 π‘šπ‘šπ‘šπ‘šπ‘šπ‘š

π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘š π‘šπ‘šπ‘œπ‘œ 𝑁𝑁𝑁𝑁𝑁𝑁𝑁𝑁 . 𝟐𝟐𝟎𝟎𝟐𝟐 π’Žπ’Žπ’Žπ’Žπ’Žπ’Ž 𝑡𝑡𝑡𝑡𝑡𝑡𝑡𝑡

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Concentration can also be described in terms of mass percent (mass %), which is simply the number of grams of solute per 100 grams of solution (Equation 2).

π‘šπ‘šπ‘π‘π‘šπ‘šπ‘šπ‘š π‘šπ‘šπ‘œπ‘œ π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘ π‘ π‘ π‘ π‘šπ‘š (𝑔𝑔)

Mass % = Γ— 100 % =

π‘šπ‘šπ‘π‘π‘šπ‘šπ‘šπ‘š π‘šπ‘šπ‘œπ‘œ π‘šπ‘šπ‘šπ‘šπ‘šπ‘šπ‘ π‘ π‘ π‘ π‘ π‘ π‘šπ‘šπ‘ π‘  (𝑔𝑔)

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Example 3: What is the mass percent dextrose in an aqueous solution of dextrose (C6H12O6) that was prepared by dissolving 15.3 grams of dextrose in 155.0 g of water: The total mass of the solution is 170.3 g (15.3 g + 155.0 g):

15.3g C H O

Mass % Dextrose = 6 12 6 Γ—100% = 8.98% C6H12O6 (by mass)

170.3g solution

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Diluted solutions are prepared from a stock solution. Stock solutions have a known concentration and are more concentrated than diluted solutions. Common aqueous stock solutions are prepared by dissolving a known amount of solid into a known volume of water, as described above. To prepare a diluted solution from a stock solution, the dilution equation is used:

M1V1 = M2V2

M1 = molarity of stock solution

V1 = volume of stock solution diluted

M2 = molarity of diluted solution V2 = volume of diluted solution

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Example 4: Hydrochloric acid is often purchased as a 12.0 M aqueous stock solution. A student needs 250 mL of a 3.0 M solution of hydrochloric acid. Using the dilution equation, calculate the volume of 12.0 M HCl needed to prepare the solution.

M1V1 = M2V2

(12.0 M) (V1) = (3.0 M) (250 mL)

V1 = 62.5 mL = V1 = 63 mL (corrected for significant figures)

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When expressing the concentration value, the type of lab equipment used will affect the accuracy and precision of the value. For example, when working with mass percent, the number of significant figures in the measurement will depend on the mass measurements as determined by the balance used. If in Example 3, the student instead had recorded the masses to the ones place, the result mass percent value would be 8.8% rather an 8.98%. When the concentration value depends on volume, as it does with molarity, the type and size of glassware used to measure volume will affect the accuracy and precision of the measured molarity. There are two categories of glassware used to measure volume: β€œTo Deliver” (TD) and β€œTo Contain” (TC). Graduated cylinders, burets, volumetric pipets and graduated (or Mohr) pipets are all TD; while volumetric flasks are TC. Graduated glassware, such as graduated cylinders, burets, and graduated pipets, have etched lines or calibration marks on their surface. Each calibration mark corresponds to a specific volume of liquid. Therefore, a 100-mL graduated cylinder, for example, can used to measure different volumes up to 100 mL. However, volumetric flasks and volumetric pipets contain only one calibration mark and are used to measure only one volume. As a result, all volumetric glassware is very accurate when used correctly. Your instructor will demonstrate proper technique.

When a chemical reaction is performed in a solution, stoichiometry can be used to determine the amount of reactants needed for reaction, or the amount of product(s) obtained from a reaction. Because a chemical reaction involves changes in composition, the dilution equation cannot be used, and stoichiometry is used instead.

Example 5: Consider the reaction:

2 KI(aq) + Pb(NO3)2(aq) Γ† PbI2(s) + 2 KNO3(aq)

How much 0.214 M KI solution in liters is needed to completely precipitate the Pb2+ in 0.205 L of 0.500 M Pb(NO3)2 solution?

Note: In this problem, the volume (in liters) and molarity of the Pb(NO3)2 solution are given, and the volume of KI is what we are calculating. As these are separate reactants, the dilution equation cannot be used and stoichiometry is used instead. In solution stoichiometry, the molarity of a solution is used to convert the volume of the solution to the moles of the solute:

word image 855 0.500 π‘šπ‘šπ‘šπ‘šπ‘šπ‘š 𝑃𝑃𝑃𝑃(𝑁𝑁𝑁𝑁3)2 2 π‘šπ‘šπ‘šπ‘šπ‘šπ‘š 𝐾𝐾𝐾𝐾 1 𝐿𝐿 𝐾𝐾𝐾𝐾

0.205 𝐿𝐿 𝑃𝑃𝑃𝑃(𝑁𝑁𝑁𝑁3)2 . πŸ—πŸ—πŸ—πŸ—πŸπŸ 𝐋𝐋 𝐊𝐊𝐊𝐊

1 (𝑁𝑁𝑁𝑁3 2 1 3 2

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In this lab, you will react calcium chloride with potassium carbonate to produce solid calcium carbonate. You will then use stoichiometry to determine the theoretical yield and the percent yield of the reaction. You will also prepare a percent by mass stock solution and a molar stock solution, and perform a simple dilution.

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PROCEDURE:

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Part 1 – Preparation of CaCO3(s)

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The following is a procedure that was theoretically performed by a student. Read through the procedure and answer the questions below.

  1. A 10.0 mL graduated cylinder to measure 10.0 mL of a 1.00 M CaCl2 solution into an initially empty

50.0 mL beaker.

  1. A 50.0 mL graduated cylinder was then used to measure out 25.0 mL of 0.500 M K2CO3. This K2CO3 solution was then added to the beaker containing the CaCl2 solution. The solution became cloudy, and the student concluded that a precipitate must have formed. Write a balanced chemical reaction below, including phases, and identify the chemical formula of the precipitate:

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  1. The student then collected the precipitate by filtering it using the gravity filtration apparatus below:

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word image 2471

Figure 3.1 – Gravity Filtration Apparatus (Conical Flask = Erlenmeyer Flask)

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word image 2472 Unless otherwise noted, content of University of York is licensed under CC BY 4.0

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After the solid was collected, it was dried on a hot plate three times. The following data was collected:

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Mass of solid product after first heating (g)

1.0250 g

Mass of solid product after second heating (g)

0.9723g

Mass of solid product after third heating (g)

0.9719 g

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Answer the questions below as part of Data Analysis.

  1. Is the solid product dry? How do you know?

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  1. What is the actual yield of the product?

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  1. Using stoichiometry, calculate the limiting reactant, and the theoretical yield (the mass of CaCO3 product) for the reaction.

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  1. Determine the percent yield of the reaction.

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Part 2 – Preparing a Solution

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For Part 2, you will need to complete the following steps at the following website: https://phet.colorado.edu/sims/html/concentration/latest/concentration_en.html

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  1. On the Phet simulation website (link above), you should see a tank filled with 0.5 L of water, as well as a shaker to add solid, a valve to add water, a valve to empty the tank, a button for evaporation of the solvent, a tool to measure the concentration, and a button to change the solute.
  2. On the upper right hand corner of your screen, change the solute from β€œDrink mix” to β€œNickel(II) chloride.”
  3. Using your cursor, move the circular concentration measuring tool so that it sits on top of the tank. The concentration tool should now read 0.000 mol/L.
  4. Using your cursor to move the shaker (labeled NiCl2) around until the concentration tool reaches between 0.300 mol/L to 0.400 mol/L. It doesn’t have to be exact!

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Write down the concentration you measured: _______________________

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  1. Using the concentration measured and the volume of solution on your screen, determine the mass of nickel(II) chloride solute that you added to your solvent. Show your calculation below:

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  1. If you add additional 0.50 L of water to the solution, what would the new volume of the solution be? Without doing any calculations, predict whether the concentration of the solution will increase or decrease. Explain your answer.

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Calculate the theoretical concentration of this new solution using the new volume and the dilution equation (M1V1 = M2V2). Show your work below:

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  1. Now determine the experimental concentration. Using your cursor, click and drag the valve on the top left hand side of your screen to add an additional 0.5 L of water. Write down your observations about what happened to the color of the solution after adding the water:

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  1. Read the experimental concentration using the concentration tool.

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Write down the concentration you measured: _______________________

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  1. Calculate the percent error of your solution concentration. Show your work below:

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  1. Now refresh your screen by clicking on the yellow circular arrow on the bottom right hand side of your screen. You should see your screen go back to its standard settings.
  2. You are going to now determine the concentration of an unknown stock solution by preparing a dilute solution. Once again, change your solute to β€œNickel(II) chloride.” Right under where you change your solute, click on solution (next to a little dropper). You should now see a green dropper above the tank of water.
  3. Click on the dropper button to add 0.20 L of your stock solution to the water tank. What is the total volume of the dilute solution? Write down your observations about what happened to the color of the solution in the tank after adding the stock solution:

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Volume of dilute solution: ________________ Observations:

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  1. Using the concentration tool, measure the concentration of this new dilute solution.

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Write down the concentration you measured: _______________________

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  1. Use the dilution equation (M1V1 = M2V2) to calculate the concentration of the stock solution in the dropper bottle. Show your work below:

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  1. On this same screen, use the Evaporation tool on the bottom of your screen to bring the volume back down to 0.50 L. What is the concentration of this new solution? Write down your observations about the color change of this new solution.

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Concentration of this new solution: _________________________

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Observations:

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Did the concentration increase or decrease? Why do you think the concentration of the solution didn’t go back to what it was before? (Hint: what does Evaporation mean? Does it change the number of solute particles or volume of solvent?)

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POST-LABORATORY QUESTIONS FOR PART 2:

  1. Visually, how can you tell that the nickel(II) chloride solution has been diluted?

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  1. How can you change the concentration of a solution, yet keep the number of moles of solute the same? Explain. (Hint: what happened when you used the Evaporation tool?)

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  1. If you were to prepare your solutions in the lab, which tools would be best to use to get the most accurate concentration: a beaker, graduated cylinder, or volumetric flask? Why?

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ADDITIONAL EXERCISES

  1. Determine the mass (in grams) of sodium chloride needed to prepare a 500.0 mL of a 2.0 M solution.

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  1. Determine the concentration of a diluted solution of sodium chloride that was prepared by diluting 10.0 mL of a 4.5 M solution of sodium chloride to 250.0 mL.

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  1. Determine mass percent of a solution of sucrose (C12H22O11) prepared by dissolving 3.245 g of C12H22O11 in 73.5 g of water.

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  1. A student is preparing a solution of sucrose (C12H22O11) at a specific concentration. They are doing this by weighing out sucrose on a scale, placing it in a volumetric flask, dissolving it in water, and then diluting it up to the mark on the volumetric flask with water. How would the experimental (actual) molar concentration for C12H22O11 differ from the calculated concentration (higher, lower, or the same) given the following scenarios? Explain. (Hint: Compare the calculated concentrations to experimental concentrations).
    1. A student spilled some of the solid C12H22O11 before adding it to the volumetric flask.

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    1. A student added water so that is was about 5 mm higher than the indicated line on the volumetric flask.

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    1. When preparing the diluted solution, a student used the pipet bulb to blow out the last few drops in the pipet into the solution.

College of the Canyons Fall 2020

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  1. What volume of 0.08892 M HNO3 is required to react completely with 0.2352 g of potassium hydrogen phosphate?

2 HNO3(aq) + K2HPO4(aq) ⟢ H3PO4(aq) + 2 KNO3(aq)

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