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Fourth Example

In this fourth example the question involves moles. We're asked, "How many moles of magnesium would it take to react with 3.0 moles of oxygen?"
How many moles of magnesium would it take to react with 3.0 moles of oxygen?
We start with moles of oxygen, so the conversion factor will have moles of oxygen on the bottom. We want to find out how many moles of magnesium so that will go on the top. 3.0 moles O2 = ? moles Mg
Look at the relationship in the balanced equation. It says there are two moles of magnesium for every one mole of oxygen. So, two moles of magnesium on the top and one mole of oxygen on the bottom. 3.0 moles O2 x    2 moles Mg  
1 mole O2		 = ? moles Mg
Completing the calculation shows that we need 6.0 moles of magnesium to react with 3.0 moles of oxygen. 3.0 moles O2 x    2 moles Mg  
1 mole O2		 = 6.0 moles Mg

I should point out a very important thing about significant digits and the mole ratios in balanced equations. When we balance equations, we do so by thinking about how many atoms of one element react with the atoms of another element. The numbers we come up with are exact relationships. When we interpret balanced equations in terms of moles, we are still dealing with an exact relationship. Consequently, when rounding off your answer, you don't have to be concerned with the number of digits in the ratio of 2 moles of magnesium over 1 mole of oxygen. The ratio is an exact ratio. The answer will be rounded off to the number of digits shown in 3.0 moles of oxygen.

 

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