Chemistry Practice Test: Solutions and Molarity
- Solutions and Molarity
- 10 questions
- 14 minutes
This test covers ways to express concentration and how dissolved particles change a solution's properties. Tip: molarity uses liters of solution, while molality uses kilograms of solvent.
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Question 1
A student dissolves 11.7 g of NaCl (molar mass 58.5 g/mol) in enough water to make 500. mL of solution. What is the molarity?
Correct answer: D
11.7 ÷ 58.5 = 0.200 mol, and 0.200 mol ÷ 0.500 L = 0.400 M. 0.200 M is the number of moles, not yet divided by the volume in liters.
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Question 2
How many moles of solute are in 250 mL of a 0.40 M solution?
Correct answer: B
Moles = molarity × liters = 0.40 mol/L × 0.250 L = 0.10 mol. Using 250 instead of 0.250 L gives the impossible answer of 100 mol.
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Question 3
A chemist wants to dilute 10.0 mL of 12 M HCl to a concentration of 0.60 M. How much water must be added?
Correct answer: C
M1V1 = M2V2 gives a final volume of 12 × 10.0 ÷ 0.60 = 200 mL, and 200 − 10.0 = 190 mL of water is added. 200 mL is the final volume, not the water added.
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Question 4
What is the molality of a solution made by dissolving 0.50 mol of solute in 250 g of water?
Correct answer: A
Molality is moles of solute per kilogram of solvent: 0.50 mol ÷ 0.250 kg = 2.0 m. 0.125 m multiplies by the mass instead of dividing.
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Question 5
What is the percent by mass of sugar in a solution made from 15 g of sugar and 85 g of water?
Correct answer: B
Mass percent = mass of solute ÷ mass of solution × 100 = 15 ÷ (15 + 85) × 100 = 15%. 17.6% divides by the water only, not by the whole solution.
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Question 6
Which substance dissolves best in hexane, a nonpolar liquid?
Correct answer: D
"Like dissolves like": the nonpolar I2 molecule mixes well with nonpolar hexane. Ionic salts and the polar sugar molecule dissolve well in water but poorly in hexane.
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Question 7
Each solution has a concentration of 1.0 mol of solute per kilogram of water. Which solution has the lowest freezing point?
Correct answer: A
Freezing-point lowering depends on the number of dissolved particles. CaCl2 splits into 3 ions, NaCl and KBr into 2 each, and glucose stays as 1 molecule.
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Question 8
A clear solution contains more dissolved sodium acetate than water can normally hold at that temperature. When a tiny crystal is dropped in, solid quickly forms throughout the liquid. What kind of solution was it?
Correct answer: C
A supersaturated solution holds more solute than its normal limit and is unstable, so a seed crystal starts rapid crystallization. A saturated solution is already at its limit and would not suddenly form so much solid.
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Question 9
What is the concentration of sodium ions in a 0.20 M solution of sodium sulfate, Na2SO4?
Correct answer: B
Each Na2SO4 gives two Na+ ions, so [Na+] = 2 × 0.20 = 0.40 M. 0.60 M is the total concentration of all ions (Na+ plus SO42−).
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Question 10
A 500 g water sample contains 0.0015 g of fluoride. What is the fluoride concentration in parts per million (ppm)?
Correct answer: C
ppm = mass of solute ÷ mass of sample × 106 = 0.0015 ÷ 500 × 1,000,000 = 3 ppm. A misplaced decimal is the usual source of 0.3 or 30 ppm.
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