Determine the percent composition of the sodium, Na, the carbon, C, and the oxygen, O, within sodium carbonate, Na2CO3. Accomplish this by finding the molar mass of the sodium carbonate, which is about 105.99 grams/mole.
In order to determine the percent of sodium, multiply the molar mass of sodium by two, because there are two moles of sodium within sodium carbonate. This value should be about 45.98 grams/mole. Divide this value by the total mass of sodium carbonate. Then, multiply your answer by 100. The percent composition of sodium should be about 43.38 percent. In order to find the percent composition of the carbon, divide the molar mass of carbon, about 12.01 grams/mole, by the total mass of the sodium carbonate. Then multiply this value by 100. The answer should be about 11.33 percent. In order to find the percent composition of the oxygen, multiply the molar mass of the oxygen by three, because there are three moles of oxygen within sodium carbonate. You should receive an answer of about 48.00 grams/mole. Next, divide this value by the total molar mass of the sodium carbonate. After multiplying this number by 100 you should receive an answer of about 45.29 percent. These are your standard values. These should be the percent compositions present in your sample if it is pure.
Compare the percent compositions of your sample of sodium carbonate to the standard percent composition values. If the values are similar, then your sample is pure. However, if the values of the percent compositions of the elements in your sample differ extensively from those of the standard values, then your sample is not pure.
Analyze which element is not present in the proper ratio. Part of the concept of determining the purity of a sample of Na2CO3, or sodium carbonate, is understanding where the lack of purity is present. A substance can lack purity by having much more of one element present than the other two. If carbon has the highest percent composition within your sample, then your sample has a very high degree of impurity because carbon should be present in the smallest ratio. Discrepancies in the values of the sodium and oxygen are more understandable because the percent compositions of the sodium and the oxygen were more similar.