For an alloy that consists of 33.5 wt% Pb and 66.5 wt% Sn, what is the composition (a) of Pb (in at%), and (b) of Sn (in at%)? The atomic weights for Pb and Sn are 207.2 and 118.7 g/mol, respectively.

Pb: 22.4 at%

Sn: 77.6 at%

Explanation:

It is possible to find at% of Pb and Sn converting mass in moles using molar mass assuming a basis of 100g, thus:

Pb: 33.5g × (1mol / 207.2g) = 0.1617mol

Sn: 66.5g × (1mol / 118.7g) = 0.5602mol

Total moles: 0.1617mol + 0.5602mol = 0.7219mol

Composition in at%:

Pb: 0.1617mol / 0.7219mol × 100 = 22.4 at%

Sn: 0.5602mol / 0.7219mol × 100 = 77.6 at%

I hope it helps!