Molarity: Quiz

Molarity: Quiz

7 questions

1. Question 1EasyHow many moles of solute are in 250.0 mL of a 0.400 M solution?

   A0.100 molB1.60 molC100. molD0.00160 mol
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   Answer: 0.100 mol

   moles = molarity × volume = 0.400 mol/L × 0.2500 L = 0.100 mol. The answer 100. mol forgets to convert mL to L.

2. Question 2Medium7.46 g of potassium chloride, KCl, is dissolved to make 500.0 mL of solution. What is the molarity? (K = 39.10, Cl = 35.45)

   A0.100 MB0.200 MC0.0149 MD14.9 M
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   Answer: 0.200 M

   Moles = 7.46 ÷ 74.55 = 0.1001 mol. Molarity = 0.1001 ÷ 0.5000 L = 0.200 M.

3. Question 3Medium10.0 mL of 12.0 M HCl is diluted to a final volume of 250.0 mL. What is the new concentration?

   A0.480 MB0.300 MC2.08 MD12.0 M
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   Answer: 0.480 M

   $M_2 = \dfrac{M_1V_1}{V_2} = \dfrac{12.0 \times 10.0}{250.0} = 0.480$ M. Dilution keeps the moles the same but spreads them through a larger volume.

4. Question 4EasyWhen a solution is diluted with water, which quantity stays the same?

   AThe molarityBThe volumeCThe moles of soluteDThe mass of the solution
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   Answer: The moles of solute

   Adding water changes the volume and lowers the molarity, but no solute is added or removed, so the moles of solute are unchanged. That is why $M_1V_1 = M_2V_2$.

5. Question 5MediumWhat volume of 0.125 M NaOH neutralizes 20.00 mL of 0.250 M HCl?

   A10.0 mLB20.0 mLC40.0 mLD80.0 mL
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   Answer: 40.0 mL

   Moles of HCl = 0.250 × 0.02000 = 0.005000 mol. The ratio is 1 : 1, so NaOH needed = 0.005000 mol, and volume = 0.005000 ÷ 0.125 = 0.0400 L = 40.0 mL. The answer 10.0 mL puts the concentrations the wrong way round.

6. Question 6HardWhat volume of 0.200 M NaOH neutralizes 20.00 mL of 0.100 M $\ce{H2SO4}$? $\ce{H2SO4 + 2NaOH -> Na2SO4 + 2H2O}$

   A5.00 mLB10.0 mLC20.0 mLD40.0 mL
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   Answer: 20.0 mL

   Moles of $\ce{H2SO4}$ = 0.100 × 0.02000 = 0.00200 mol. The ratio is 2 : 1, so NaOH needed = 0.00400 mol, and volume = 0.00400 ÷ 0.200 = 0.0200 L = 20.0 mL. The answer 10.0 mL ignores the 2 : 1 ratio.

7. Question 7HardA student dissolves 0.100 mol of a solid in 250.0 mL of water. Why might the molarity not be exactly 0.400 M?

   AMolarity uses the volume of solution, not of water addedBThe moles of solute change when it dissolvesCMolarity must be calculated in millilitresDSolids cannot be used to make solutions
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   Answer: Molarity uses the volume of solution, not of water added

   Dissolving a solid changes the total volume slightly. To make exactly 0.400 M, dissolve the solid and then add water up to a final volume of 250.0 mL.