A 2.45-g sample of strontium completely reacts with oxygen to form 2.89 g of strontium oxide. Use this data to calculate the mass percent composition of strontium in strontium oxide.

The % Mass Composition of Sr is 74.72≅  75%

Explanation:

The balanced equation:

Sr + O2 → SrO2

Molecular mass of SrO2 = 87.6 + (16 × 2) = 119.6 gmol-1

1 mole SrO2 = 119.6gmol-1

how many mole is in 2.5 g of SrO2 = 2.5÷119.6

= 0.0209 mol

The mole ration of the equation is:

1 mole SrO2 was produced by 1 Sr ion and 1 O2 ion

SrO2: Sr ratio is 1:1

⇒0.0209 mol SrO2 = 0.0209 mole of Sr

mass of Sr  = 0.0209 × 87.6gmol-1

= 1.83g

∴ & mass of Sr in sample = × 100

= 74.72≅  75%

i believe the answer is three water molecules