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Hydrogen and oxygen combine in the ratio of 1:8 by mass to form water. How many grams of hydrogen are needed to react completely with 6.4 g of oxygen gas? Show complete working. Which law of chemical combination will govern your answer? Write the law.

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Explanation: Number of moles of water = mass of water/ molar mass.  

Molar mass of H2O = (1*2) + 16 = 18.0  

 Now we only need to calculate the number of moles of water.  

The chemical reaction is as follows  

CH4 + 2O2 ---> CO2 + 2H2O  

From the eqn, the moles of H2O = 2*moles of CH4  

Molar mass of CH4 = 12 + 4 = 16. Number of moles of methane = mass/

molar mass.   No of moles = 6.4/16 = 0.4.  So the number of mole of water = 2* 0.4 = 0.8. We already calculate its molar mass earlier.  So mass of water = molar mass * number of moles = 18 * 0.8 = 14.4

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