2022 H2 Chem P1 Q7 — Mole Solution

The answer is A (0.82).

Take 100 g of solder glass: 16 g of \(\text{B}_2\text{O}_3\) and 84 g of PbO.
\(M_\text{r}(\text{B}_2\text{O}_3) = 2(10.8) + 3(16.0) = 69.6\); \(M_\text{r}(\text{PbO}) = 207.2 + 16.0 = 223.2\)
\(n(\text{Pb}) = n(\text{PbO}) = \frac{84}{223.2} = 0.376\) mol
\(n(\text{B}) = 2 \times n(\text{B}_2\text{O}_3) = 2 \times \frac{16}{69.6} = 0.460\) mol
Pb / B = \(\frac{0.376}{0.460} = 0.82\)

• B is wrong: 0.91 does not come from the moles of Pb and B atoms worked out above.
• C is wrong: 1.22 is \(\frac{0.460}{0.376}\), the B / Pb ratio, which is the inverse of the ratio asked for.
• D is wrong: 1.64 is \(\frac{0.376}{0.230}\), the ratio of PbO to \(\text{B}_2\text{O}_3\); it forgets that each \(\text{B}_2\text{O}_3\) contains 2 B atoms.