Video Transcript
What is the percentage composition of calcium carbonate,CaCO₃? State percentages to one decimal place.
The percentage composition of a substance is a list of all the mass percentages of all the elements in that substance. The substance we have to analyse is calcium carbonate. We can figure out the elements in calcium carbonate by looking at the formula. We have Ca for calcium, C for carbon, and O for oxygen. Basically, what we’re going to do is think about having an amount of calcium carbonate and thinking about separating it out into its elements and seeing their mass.
For each element, we’ll work out the mass percentage by taking the mass of that element, dividing it by the mass of sample, and multiplying it by 100 percent. By the end, all the mass percentages should sum to 100 percent because we will have accounted for all the mass of the sample. Before we can start figuring out what the mass percentages are, we need to know the relative atomic masses of each element. The relative atomic mass tells us the relative mass of an average atom of that element. You can find these values on the periodic table. The value for calcium is 40.078. The value for carbon is 12.011. And the value for oxygen is 15.99.
The next thing we need to account for is the number of each element in the formula. Per calcium carbonate formula, we have one calcium, one carbon, and three oxygen. We don’t really need to worry about the fact that calcium in this case is an ion. We’re thinking about the mass of the element we could separate from the sample.
The next thing we need to do is think about what mass contribution each element makes to the total mass of the substance. We do this by taking the number of that element and multiply it by its relative atomic mass. This gives us relative mass contributions of 40.078 for calcium, 12.011 for carbon, and 47.997 for oxygen. Remember we have three oxygens per formula unit of calcium carbonate. If we sum all these together, we end up with the relative formula mass of calcium carbonate, giving us a value of 100.086.
The next step is to work out the mass percentage. Now at this point you could think of an amount of calcium carbonate and then do the calculations that way. But you don’t really need to. What we have is the mass contribution of each element and the relative formula mass of the compound. All we need to do is divide one by the other and multiply it by 100 percent to get our mass percentage.
So the mass percentage of calcium in calcium carbonate is 40.078 divided by 100.086 all multiplied by 100 percent. This gives a value of about 40 percent. The mass percentage for carbon is 12.011 divided by 100.086 multiplied by 100 percent which gives us a value of about 12 percent. And the value for oxygen is about 48 percent. If you add these three numbers up, you’ll see that they sum to 100 percent. If they didn’t, it would suggest that a mistake has been made earlier on.
The question asked us to state our percentages to one decimal place. So our calcium mass percentage is 40.0 percent. Our carbon percentage is 12.0 percent. And our oxygen mass percentage is 48.0 percent. So the final answer for the percentage composition of calcium carbonate is calcium: 40 percent, carbon: 12 percent, and oxygen: 48 percent.
