Volume of 3 M NaOH (formula weight 40 g mol⁻¹ ) which can be prepared from 84 g of NaOH is __________ × 10⁻¹ dm³.

Solution

Chemistry ma’am · 3 steps

Turn the mass into moles first, then let molarity tell you how many litres those moles can fill.

1. Moles of NaOH. Divide the mass by the formula weight: n = 84/40 = 2.1 mol.

   V = n/M = (2.1 mol)/(3 mol dm⁻³) = 0.7 dm³

2. Match the blank. Molarity is moles per litre, and 1 L = 1 dm³. The blank asks for the volume in units of 10⁻¹ dm³, so 0.7 dm³ = 7 × 10⁻¹ dm³.
3. So. The volume of 3 M NaOH that can be prepared is 7 × 10⁻¹ dm³, so the answer is 7.

The common slip: 0.7. Writing the volume in dm³ straight into the blank gives 0.7, but the blank is in units of 10⁻¹ dm³, so the entry is 7.

The idea to keep. For any molarity question, go mass to moles, then V = n/M gives litres, which are the same as dm³. Before you write the answer, check which power of ten the blank asks for.

Answer from the NTA final answer key. Solution by PrepWiser.