This is MSJChem. In this video I'll be looking at calculating percentage purity. Percentage purity is the percentage of a pure compound in an impure sample. Here we have the equation for calculating percentage purity.
So it's the mass of the pure compound in the sample divided by the total mass of the impure sample multiplied by 100. So next we look at a simple example. A 150 gram sample of copper ore contains 87.3 grams of pure copper. Calculate the percentage purity. So to calculate the percentage purity, we divide the mass of the pure copper, which is 87.3 grams, by the total mass of the impure sample, which is 150 grams, and then we multiply by 100. And this gives us 58.2%.
Next. Next we look at some more complicated examples. 5 grams of an impure sample of hydrated ethane diolic acid was dissolved in water to make 1 decimetre cubed of solution.
25 cm3 samples of this solution were titrated against a 0.1 mol per decimetre solution of sodium hydroxide using a suitable indicator. The average volume of the titer was 14 cm3. Using this information, we'll calculate the percentage purity of the hydrated ethane dioic acid. So we'll start by looking at the balanced equation for the reaction. One mole of ethane dioic acid reacts with two moles of sodium hydroxide to form one mole of salt and two moles of water.
Note that the ethane dioic acid reacts with the sodium hydroxide in a one to two ratio. The first step is to calculate the amount in moles of sodium hydroxide in the average titer. So we use the equation N equals Cv.
The concentration of sodium hydroxide is 0.1 mole per decimeter. The average titer was 14.0 cm3 which we convert to decimeters cubed by dividing by 1000. This gives us 1.40 times 10 to the negative 3 moles. The second step is to calculate the amount in moles of ethane dioic acid that reacts with the sodium hydroxide. The molar ratio of ethane dioic acid to sodium hydroxide is a 1 to 2 ratio, so we divide the amount in moles of sodium hydroxide by 2, which gives us 7.00 times 10 to the negative 4 moles. Next we calculate the amount in moles of ethane dioic acid in 1 decimeter cubed of solution.
So we multiply the amount in moles in 25 cm3 of solution by 1000 over 25, which gives us 2.80 times 10 to the negative 2 moles. Next, we convert amount in moles to mass in grams. So we multiply the amount in moles of ethane dioic acid in 1 dm3 of solution by the molar mass of ethane dioic acid, which gives us 3.53 g.
And finally we'll calculate the percentage purity. So we divide the mass of the pure ethane dioic acid by the total mass of the impure sample. We then multiply by 100 to give us 70.6%.
Next we look at one more example. 10 grams of chalk which is impure calcium carbonate was reacted with excess hydrochloric acid. So here we have the equation for the reaction.
1 mole of calcium carbonate reacts with 2 moles of hydrochloric acid to form 1 mole of salt, 1 mole of carbon dioxide and 1 mole of water. We are also told that 2.13 decimetres cubed of carbon dioxide was produced at STP in the reaction. The first step is to calculate the amount in moles of carbon dioxide produced in the reaction.
So we divide the volume of carbon dioxide produced in the reaction by the molar volume of a gas at STP which is 22.7 decimeters cubed. This gives us 9.38 times 10 to the negative 2 moles. If we look at the molar ratio of carbon dioxide to calcium carbonate we can see it's a 1 to 1 ratio.
Because calcium carbonate is the limiting reactant 9.38 times 10 to the negative 2 moles of calcium carbonate must have reacted to produce the same amount in moles of carbon dioxide. So next we can calculate the mass of calcium carbonate that reacted with the excess hydrochloric acid. So we multiply the amount in moles by the molar mass of calcium carbonate, which is 100.09 grams per mole. And this gives us 9.38 grams.
And finally, we calculate the percentage purity. So it's the mass of the pure sample of calcium carbonate, which is 9.38 grams, by the total mass of the impure sample, which was 10.00 grams. We then multiply this by 100 to give us 93. point eight percent