To prepare a 1.0 M NaCl solution, you dissolve 1 mol of NaCl in which of the following?
AExactly 1.0 L of water
BExactly 1.0 kg of water
CEnough water so that the total volume of solution equals 1.0 L
DAny amount of water, then adjust pH to 7
Molarity is defined as moles of solute per liter of *solution*, not per liter of solvent. You dissolve the solute, then add solvent until the total solution volume reaches 1.0 L. Adding 1 mol to 1.0 L of water would give slightly more than 1.0 L of solution, yielding a molarity slightly less than 1.0 M.
Question 2 True / False
Molality is preferred over molarity for colligative property calculations because molality does not change when temperature changes.
TTrue
FFalse
Answer: True
Molality is defined as moles of solute per kilogram of *solvent* (a mass). Mass does not change with temperature. Molarity uses solution *volume*, which expands or contracts as temperature changes, so the same solution has a slightly different molarity at different temperatures. For colligative properties, which depend only on the ratio of solute to solvent particles, molality is the temperature-independent choice.
Question 3 Short Answer
A student dilutes 25.0 mL of a 4.0 M HCl solution to a total volume of 100.0 mL. What is the concentration of the diluted solution?
Think about your answer, then reveal below.
Model answer: 1.0 M
Dilution conserves moles of solute: M₁V₁ = M₂V₂. Substituting: (4.0 M)(25.0 mL) = M₂(100.0 mL), so M₂ = 100/100 = 1.0 M. The moles of HCl have not changed — they are now spread through a four-times-larger volume.