In the formula for water (h2o), what does the subscript 2 indicate?

Asample of sea water contains 6.28g of sodium chloride per litre of solution. how many milligrams of sodium chloride would be contained in 15.0ml of this solution?

How many liters of oxygen gas, at standardtemperature and pressure, will react with 35.8 grams ofiron metal? 4 fe (s) + 3 o2 (g) → 2 fe2o3 (s)

4786 joules of heat are transferred to a 89.0 gramsample of an unknown material, with an initialtemperature of 23.0°c. what is the specific heat of thematerialif the final temperature is 89.5 °c?