Chemistry
Ms. Boon
January 13, 2013
Benchmark Review Quiz Answers and Explanations
Standard 6a
When NaCl dissolves in water, water is the
(a) solvent.
(c) solute.
(b) suspension.
(d) colloid.
Solutions are made up of a solvent and one or more solutes. The solvent is the substance in which the solutes are dissolved.
Water is considered the universal solvent because so many different substances dissolve in water. In this case, NaCl (salt) is
dissolved in water. Water is the solvent and salt is the solute.
In an aqueous solution, the dissolved particles are the
(a) solute.
(c) solubility.
(b) solvent.
(d) solvation.
To answer this question correctly, you must remember that a solution contains a solvent and one or more solute. The solute
is the substance that dissolves in the solvent. There is usually more solvent than solute. In this question, the solution is an
aqueous solution meaning that water is the solvent and whatever is dissolved in it is the solute.
Dentists use mixtures of liquid mercury (Hg) and solid silver (Ag) for fillings in teeth. A typical composition is 4.0 grams
mercury and 50.0 grams of silver. The mercury would be called the
(a) solvent.
(c) solute.
(b) solution.
(d) dissolving medium.
In a solution, there is more solvent than solute. In this case, there is more Ag and less Hg. Therefore, Hg is the solute.
The salt that is dissolved in water to make a solution is called the
(a) sediment.
(c) solute.
(b) suspension.
(d) solvent.
In a salt water solution, the salt dissolves in the water. Therefore, the salt is the solute and the water is the solvent.
Standard 2d
Which statement best explains why sugar dissolves in water?
(a) The temperature increases sufficiently to cause atomic motion.
(b) The temperature increases sufficiently to break the interatomic bonds.
(c) The intermolecular forces between sugar and water molecules are stronger than those between the sugar molecules.
(d) The potential energy of the atoms overcomes the intermolecular forces.
In order for a solute to be dissolved in a solvent, the attractive forces between the solute and solvent particles must be great
enough to overcome the attractive forces within the pure solvent and pure solute. These attractive forces are called
intermolecular forces. “A” is incorrect because all of the atoms are already moving. “B” is incorrect because no interatomic
bonds are broken. “D” is incorrect because it is unclear and vague. “C” is the only answer that explains what has to happen
between the water and sugar molecules.
A chemist wants to estimate the strength of the intermolecular forces for a chemical compound. Which property of the
compound will give the chemist the best estimate of the intermolecular forces?
(a) density
(c) melting point
(b) solubility
(d) molecular mass
Intermolecular forces are forces of attraction between molecules. A substance with strong intermolecular forces will have a
higher melting point because it will take more energy to break the intermolecular forces that are keeping the substance in the
solid state. Substances with weak intermolecular forces is more likely to be a liquid or gas at a lower temperature meaning
that it must have a lower melting point. “B” is incorrect because the type of intermolecular force at work in a substance will
help determine solubility in a specific solvent, but the chemist would also have to take into account the “like dissolves like”
rule.
Consider 2 covalent solids:
Compound
Solid X
Molecular Mass
65
Solid Y
68
Chemistry
Ms. Boon
January 13, 2013
(g/mole)
Melting Point (°C)
85
42
Which accounts for the lower melting point of solid Y compared to solid X?
(a) Y has a higher molecular mass
(b) Y has weaker covalent bonds
(c) Y has stronger intermolecular forces
(d) Y has weaker intermolecular forces
Substance Y has a lower melting point, so it must have weaker intermolecular forces. (See explanation to previous question.)
“B” is incorrect because this answer implies that melting breaks covalent bonds and this is not true. For example, when ice
melts, the molecules are still covalently bonded as H2O.
Which of the following is true of a liquid?
(a) It has a definite volume and a definite shape.
(b) It has a definite volume but an indefinite shape.
(c) It has a definite shape but an indefinite volume.
(d) It has neither a definite volume nor a definite shape.
To answer this question, think about a substance in the solid, liquid, and gas phases. For example water – when it is solid it
has a definite shape and volume (size or amount), when it is water it fills the bottom of its container (indefinite shape) but it
has a definite volume (amount – we can say we have 1 Liter of water), and water vapor (gas) has indefinite shape and volume
because it spreads out to fill its container.
Standard 6d
If 36 mL of pure alcohol is diluted with water with a total volume of 80 mL, the solution is what percent alcohol by volume?
(a) 36%
(c) 45%
(b) 44%
(d) 82%
(36 mL/80 mL) x 100% = 45% Note that the question tells you that the total volume is 80 mL so that you do not need to add
36 + 80.
117 grams of sodium chloride (NaCl) are dissolved to make 1000 mL of solution. What is the molarity (M) of the NaCl?
(a) 0.117 M
(c) 2.0 M
(b) 0.20 M
(d) 11.7 M
You first need to notice that this question asks for molarity. This is moles/Liter. Convert 117 g into moles. Divide by the molar
mass of NaCl. 117/58=2 mol NaCl. Then 1000 mL = 1 L. So the molarity is 2 mol/L = 2 M
36 milligrams (mg) of toxin were detected in 3 liters (L) water. What is the concentration of the toxin?
(a) 12 ppm
(c) 108 ppm
(b) 36 ppm
(d) 12,000 ppm
36 mg = 0.036 g and 3 L = 3 kg = 3000 g
0.036 g x 1,000,000 = 12 ppm
3000 g
A solution contains 11.5 g of ethanol (C2H5OH) dissolved in 750 mL of water. The molar concentration is _________.
(a) 0.015 M
(c) 0.333 M
(b) 0.0033 M
(d) 1.5 M
This is another molarity problem. You must convert from grams ethanol to moles ethanol. The molar mass of ethanol is
46g/mol. Divide 11.5 g/46 g/mol = 0.25 mol
750 mL = 0.750 L
0.25 mol = x mol
0.750 L
1L
x = 0.333 mol/L = 0.333 M