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Moles

Calculating the Mass of a Product from a Reaction.

Example 2 (see example 1).
What mass of sodium oxide will be formed
by burning 20 g  of sodium in air?

1) Write the equation for the reaction.

sodium + oxygen  sodium oxide.
4Na(s)  +    O2(g)              2Na
2O(s)

Use the big numbers to find the proportion of reactant to product.
4Na makes 2Na
2O, so 2Na makes 1Na2O,
so the proportion is 2 to 1.

2) Divide the RFM of the product
by the RAM of the reactant.

RFM of Na2O is (2 x 23) + (1 x 16)  =  62.
RAM of Na = 23.
62 ÷ 23 = 2·696.

3) Multiply  the number in 2 by both the
proportion and the mass in grams of the reactant.

The proportion is 2 to 1, or 1 to 0·5.
The mass of reactant is 20 g.
The mass of product is 2·696 x 0·5 x 20 g,
= 26·96 g.
So, burning 20 g  of sodium in air will make
26·96 g  of sodium oxide.

See the previous page for the basic method.

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