Question

Which solution is the most concentrated?
2.0 mL of 10 M H2SO4, where H2SO4 has a molar mass of 98 g/mol
5.0 mL of 1.0 M PbSO4, where PbSO4 has a molar mass of 303 g/mol
2.0 mL of 10.5 M H2O2, where H2O2 has a molar mass of 34 g/mol
100 mL of 10 M NaCl, where NaCl has a molar mass of 58 g/mol

Answer 1

It depends what you mean by “most concentrated.”

If you mean highest molarity (mol·L−1): compare the given M values directly. 10.5 M H2O2 is the most concentrated (10.5 M > 10 M > 1.0 M).

If you mean which given sample contains the most solute (moles or grams) given the listed volumes, compute moles = M·V and mass = moles·molar mass:

- 2.0 mL of 10 M H2SO4: moles = 10 × 0.002 = 0.020 mol → mass = 0.020×98 = 1.96 g
- 5.0 mL of 1.0 M PbSO4: moles = 1.0 × 0.005 = 0.005 mol → mass = 0.005×303 = 1.515 g
- 2.0 mL of 10.5 M H2O2: moles = 10.5 × 0.002 = 0.021 mol → mass = 0.021×34 = 0.714 g
- 100 mL of 10 M NaCl: moles = 10 × 0.100 = 1.00 mol → mass = 1.00×58 = 58 g

So the 100 mL of 10 M NaCl contains the greatest amount of solute (1.00 mol, 58 g), but the most concentrated solution by molarity is the 2.0 mL of 10.5 M H2O2.