Answer:

64.6% of Br and 35.4% of Sr

Explanation:

The percentage composition is obtained defining the percentage in mass of each atom in a molecule.

To solve this question we must find the mass of each atom in 1 mole:

Molar mass Br: 79.904g/mol

Molar mass Sr: 87.62g/mol

The molar mass of SrBr2 is:

79.904g/mol*2 + 87.62g/mol =

159.808g/mol + 87.62g/mol =

247.428g/mol

The percentage of Br is:

79.904g/mol*2 = 159.808g/mol / 247.428g/mol * 100

= 64.6% of Br

And percentage of Sr is:

87.62g/mol / 247.428g/mol * 100

= 35.4% of Sr