A sample of 41.7 g of phosphorus pentachloride (208.5 g/mol) is hydrolysed completely by excess…

Step 1. Moles of PCl5. $\dfrac{41.7\ \text{g}}{208.5\ \text{g/mol}} = 0.200\ \text{mol}$.

Step 2. Complete hydrolysis. $\text{PCl}_{5}\ +\ 4\text{H}_{2}\text{O}\ \rightarrow \ \text{H}_{3}\text{PO}_{4}\ +\ 5\text{HCl}$. One mole gives 5 mol of HCl, so $0.200 \times 5 = 1.00\ \text{mol}$ of hydrogen chloride and $0.200\ \text{mol}$ of phosphoric acid.

Step 3. Sodium hydroxide. Hydrogen chloride needs $1.00\ \text{mol}$ of base. Phosphoric acid is tribasic and needs $3 \times 0.200 = 0.600\ \text{mol}$. The total is $1.60\ \text{mol}$.

Step 4. First step only. $\text{PCl}_{5}\ +\ \text{H}_{2}\text{O}\ \rightarrow \ \text{POCl}_{3}\ +\ 2\text{HCl}$ gives $0.200\ \text{mol}$ of $\text{POCl}_{3}$, with a mass of $0.200 \times 153.5 = 30.7\ \text{g}$.