Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%), 87 (abundance of 7.0%), and 88 (abundance of 82.6%). 2. Calculate the atomic mass of strontium. 3. What is the percentage composition of sodium dihydrogen phosphate? 4. An organic substance consists of 54.5% carbon, 9.1% hydrogen and 36.4% oxygen. What is the substance's empirical formula? 5. Copper(II) nitrate decomposes on heating to produce copper (II) oxide, nitrogen dioxide and oxygen gas. What mass of copper(II) nitrate must decompose to produce 100 mL of oxygen at STP?