Which compound has the highest precent composition by mass of strontium?
a) srcl2
b) sri2
c) sro
d) srs

First find the molar masses of all of the compounds:
SrCl2 = 158.53g/mol
SrI2 = 341.43g/mol
SrO = 103.62g/mol
SrS = 119.68g/mo

Then find the molar mass of Sr in each of the compounds. Since there is only one Sr in every compound, the mass will be the same: 87.62g/mol Sr

Now divide the molar mass of Sr by each of compounds' masses:
SrCl2 = (87.62)/(158.53) = 0.553
SrI2 = (87.62)/(341.43) = 0.257
SrO = (87.62)/(103.62) = 0.846
SrS = (87.62)/(119.68) = 0.732

Now multiply each of the results by 100 to get the percent composition of Sr by mass:
(0.553)(100) = 55.3% Sr in SrCl2
(0.257)(100) = 25.7% Sr in SrI2
(0.846)(100) = 84.6% Sr in SrO
(0.732)(100) = 73.2% Sr in SrS

Thus SrO has the highest percent composition by mass of Sr