What is the approximate percentage by mass of oxygen in Zn(NO3)2? A) 17 percent, B) 38 percent, C) 51 percent, D) 66 percent, or E) 95 percent.
In the question, we have the formula for zinc nitrate. And we can see that oxygen forms a part of the nitrate anions. Our job is to work out what proportion of the mass of zinc nitrate is due to the oxygen. So what we need to work out is the mass of the oxygen and then the mass of the zinc nitrate, divide one by the other and multiply by 100 percent. To do this, we can use the atomic masses of the elements.
An atom of zinc has a mass of about 65 unified atomic mass units. You might see u written as amu. They’re slightly different units, but they have the same effect in this circumstance. We can use the rounded 65u in this case because we’re only looking for the approximate percentage by mass of oxygen. Nitrogen has an atomic mass of 14 unified atomic mass units. And an atom of oxygen has a mass of 16 unified atomic mass units. In a unit of zinc nitrate, we have one zinc, two nitrogens, and six oxygens.
Then, what we can do is take the atomic mass and multiply it by the number of that element we find in the formula. This will give us the mass contribution for each element: 65 unified atomic mass units from zinc, 28 from nitrogen, and 96 from oxygen. So we’ve already got the first term for our equation, which is the mass that oxygen contributes to a unit of zinc nitrate. All we need to do is sum up the contributions from all the elements to get the other term, the mass of a unit of zinc nitrate in unified atomic mass units, 189.
To get the approximate percentage by mass of oxygen in zinc nitrate, we take the mass due to oxygen and divide it by the mass of an entire unit. We can simplify the fraction to 32 over 63. That’s too high for 17 percent, too low for 95 percent, too high for 38 percent, and too low for 66 percent, leaving us with the estimate of 51 percent. If we used a calculator, we’d end up with a value of 50.7937 percent.