How many grams of water are needed to form 21.8 grams of oxygen gas?

24.5g

Explanation:

Given parameters:

Mass of oxygen gas = 21.8g

Unknown:

Mass of water  = ?

Solution:

This problem deals with the decomposition of water;

    Reaction equation;

                  2H₂O →  2H₂  +  O₂

2 mole of water will produce 1 mole of oxygen gas

First find the number of mole of the oxygen gas given;

          Number of moles  =

    molar mass of O₂   = 2(16) = 32g/mol

  Number of moles of O₂  =   = 0.68moles

    1 mole of oxygen gas will be produced from 2 mole of water

     0.68moles of oxygen gas will be produce from  0.68 x 2 moles of water;   = 1.36moles of water

Mass of water = number of moles x molar mass

  Molar mass of water = 2(1) + 16  = 18g/mol

Mass of water  = 1.36 x 18  = 24.5g