Question

When 5 g of calcium is burnt in 2 g of oxygen, then 7 g of calcium oxide is produced.
What mass of calcium oxide will be produced when 5 g of calcium is burnt in 20 g of oxygen, which law of chemical combination will govern your answer? ​

Answer 1

The reaction between oxygen and calcium is as follows:

2Ca  +  O₂   ----------------->   2CaO

The mole ratio of the above equation is :

2:1:2  which means that every 2 moles of calcium requires 1 mole of oxygen to form 2 moles of calcium oxide.

Calculate the actual moles of both oxygen and calcium to find the limiting reagent in the equation as follows;

 moles of oxygen :

moles = mass/molar mass     ----->     2/32 = 0.0625 moles

moles of calcium:

moles = mass/molar mass --------->  5/40 = 0.25 moles

When 20 grams of oxygen is used:

find moles of 20g of oxygen

moles = mass/molar mass -----------> 20/32   = 0.625 moles

Therefore calcium is the limiting reagent --we will use its actual moles(which is 0.25) to calculate the mass of CaO produced.

The mole ratio between Ca used and CaO produced from the equation above is: 2:2 which is 1 to 1. This means that 1 mole of Ca will give 1 mole of CaO. 

Since the mole of Ca is 0.125, then mole of CaO produced will also be 0.125.

If the moles of CaO produced is 0.125, calculate its mass as follows:

Mass = moles × molar mass

molar mass of CaO is 56

Therefore mass produced = 0.125 × 56

                                          = 7g

The mass of calcium oxide produced will be 7g(all the calcium will be used up but the oxygen will remain in excess)

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