What is the percent composition (by mass) of MgBr2?

13.2% Mg, 86.8% Br

Explanation:

Mg has a molar mass of 24.31g/mol.

Bromine has a molar mass of 79.90 g/mol.

There is one mole of Mg and two moles of Br in the compound:

79.90x2 + 24.31 = 184.11

So the total molar mass of MgBr2 is 184.11.

A percent tells us a part of a whole, so we will divide each part of the mass into the whole mass.

24.31g Mg / 184.11g MgBr2 = .132

159.8g Br / 184.11g MgBr2 = .868

Multiply these decimals by 100 to make them into a percent.

.132 x 100 = 13.2% Mg

.868 x 100 = 86.8% Br

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