Question

Select the correct mass for each of the samples described below.

3.01 × 1023 molecules H2O

8.32 × 1020 formula units CaBr2

1.93 × 1026 molecules XeF6

Answer 1

Use n = N/NA and m = n·M (NA = 6.022×10^23 mol^-1).

1) 3.01×10^23 H2O
- n = 3.01×10^23 / 6.022×10^23 = 0.500 mol
- M(H2O) = 18.02 g·mol^-1 → m = 0.500·18.02 = 9.01 g

2) 8.32×10^20 CaBr2 formula units
- n = 8.32×10^20 / 6.022×10^23 = 1.381×10^-3 mol
- M(CaBr2) = 40.08 + 2·79.90 = 199.88 g·mol^-1 → m = 1.381×10^-3·199.88 = 0.276 g

3) 1.93×10^26 XeF6
- n = 1.93×10^26 / 6.022×10^23 = 320.6 mol
- M(XeF6) = 131.29 + 6·19.00 = 245.29 g·mol^-1 → m = 320.6·245.29 = 7.86×10^4 g = 78.6 kg