Chemistry: Solution Chemistry: Molarity and Dilution Worksheet

Question 1

A student dissolves 0.50 mol of NaCl in enough water to make 2.0 L of solution. What is the molarity of the NaCl solution?

Accepted Answer

The molarity is 0.25 M NaCl because molarity equals moles of solute divided by liters of solution: 0.50 mol divided by 2.0 L equals 0.25 mol/L.

Question 2

A solution is made by dissolving 20.2 g of KNO3 in enough water to make 500.0 mL of solution. The molar mass of KNO3 is 101.1 g/mol. What is the molarity?

Accepted Answer

The molarity is 0.400 M KNO3. First, 20.2 g divided by 101.1 g/mol equals 0.1998 mol KNO3. Then, 500.0 mL equals 0.5000 L, so 0.1998 mol divided by 0.5000 L equals 0.400 M.

Question 3

How many grams of NaCl are needed to prepare 250.0 mL of a 0.150 M NaCl solution? The molar mass of NaCl is 58.44 g/mol.

Accepted Answer

You need 2.19 g of NaCl. The moles needed are 0.150 mol/L times 0.2500 L, which equals 0.0375 mol. The mass is 0.0375 mol times 58.44 g/mol, which equals 2.19 g.

Question 4

A lab needs 500.0 mL of 0.250 M HCl made from a 2.00 M HCl stock solution. What volume of the stock solution is needed?

Accepted Answer

The lab needs 62.5 mL of the 2.00 M HCl stock solution. Using M1V1 = M2V2, V1 = (0.250 M times 500.0 mL) divided by 2.00 M, which equals 62.5 mL.

Question 5

A student dilutes 100.0 mL of 0.750 M CuSO4 solution to a final volume of 600.0 mL. What is the final molarity?

Accepted Answer

The final molarity is 0.125 M CuSO4. Using M1V1 = M2V2, M2 = (0.750 M times 100.0 mL) divided by 600.0 mL, which equals 0.125 M.

Question 6

A 50.0 mL sample of 0.200 M NaOH is diluted by adding water until the final volume is 200.0 mL. What is the new concentration of NaOH?

Accepted Answer

The new concentration is 0.0500 M NaOH. The moles of NaOH stay the same during dilution, so M2 = (0.200 M times 50.0 mL) divided by 200.0 mL, which equals 0.0500 M.

Question 7

Explain why a chemist who prepares a solution from a solid should dissolve the solid in some water first, then add water until the solution reaches the final volume mark.

Accepted Answer

The chemist should dissolve the solid first and then dilute to the final volume because the solid takes up space and may change the total volume. Adding a fixed amount of water first would not guarantee the correct final solution volume.

Question 8

Which solution contains more moles of solute: 0.300 L of 0.500 M glucose or 0.200 L of 0.800 M glucose? Show your work.

Accepted Answer

The 0.300 L of 0.500 M glucose contains more solute. It has 0.500 mol/L times 0.300 L, which equals 0.150 mol. The other solution has 0.800 mol/L times 0.200 L, which equals 0.160 mol, so the 0.200 L sample actually contains more solute.

Question 9

A stock solution has a concentration of 6.00 M. A student transfers 25.0 mL of this stock solution into a 250.0 mL volumetric flask and fills to the mark with water. What is the concentration of the diluted solution?

Accepted Answer

The concentration of the diluted solution is 0.600 M. Using M1V1 = M2V2, M2 = (6.00 M times 25.0 mL) divided by 250.0 mL, which equals 0.600 M.

Question 10

A solution contains 0.0750 mol of CaCl2 in 125 mL of solution. What is the molarity of CaCl2?

Accepted Answer

The molarity is 0.600 M CaCl2. The volume is 125 mL, or 0.125 L, so 0.0750 mol divided by 0.125 L equals 0.600 M.

Question 11

How many milliliters of a 0.400 M Na2CO3 solution contain 0.0200 mol of Na2CO3?

Accepted Answer

The required volume is 50.0 mL. Since volume equals moles divided by molarity, 0.0200 mol divided by 0.400 mol/L equals 0.0500 L, which is 50.0 mL.

Question 12

A student starts with 1.00 M food dye solution. The student makes a 1:10 dilution by placing 10.0 mL of the dye into a 100.0 mL volumetric flask and filling to the mark. Then the student repeats the same 1:10 dilution using the first diluted solution. What is the final concentration after the second dilution?

Accepted Answer

The final concentration is 0.0100 M. The first 1:10 dilution changes 1.00 M to 0.100 M, and the second 1:10 dilution changes 0.100 M to 0.0100 M.

Question 13

A 2.50 M solution is diluted to a final volume of 1.00 L. If 125 mL of the stock solution was used, what is the final concentration?

Accepted Answer

The final concentration is 0.313 M. Using M1V1 = M2V2, M2 = (2.50 M times 125 mL) divided by 1000 mL, which equals 0.3125 M and rounds to 0.313 M.

Question 14

A student needs to prepare 100.0 mL of 0.250 M MgCl2 solution from solid MgCl2. The molar mass of MgCl2 is 95.21 g/mol. What mass of MgCl2 should the student measure?

Accepted Answer

The student should measure 2.38 g of MgCl2. The moles needed are 0.250 mol/L times 0.1000 L, which equals 0.0250 mol. The mass is 0.0250 mol times 95.21 g/mol, which equals 2.38 g.

Question 15

A student writes, 'To make 1.00 L of 0.100 M solution, add 1.00 L of water to 0.100 mol of solute.' Explain what is wrong with this statement and write a corrected version.

Accepted Answer

The statement is wrong because the final solution volume should be 1.00 L, not the amount of water added. A corrected version is: dissolve 0.100 mol of solute in some water, then add water until the total solution volume is 1.00 L.

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Chemistry: Solution Chemistry: Molarity and Dilution

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