Calculate the formula mass of Mg(NO₃)₂ to the nearest whole number. A) 54, B) 86, C) 134, D) 148, or E) 172.
Formula mass is the mass of a single formula unit. That means a single unit of Mg(NO₃)₂, otherwise known as magnesium nitrate. In order to work out the formula mass of magnesium nitrate, we need the atomic masses of the elements that make it up. The question says to give our answer to the nearest whole number. So we can work with rounded atomic masses because they’re pretty close to round numbers anyway.
The atomic mass of magnesium is 24. The atomic mass of nitrogen is 14. And the atomic mass of oxygen is 16. In one formula unit of magnesium nitrate, we have one magnesium ion and two nitrate ions, which means we have two lots of nitrogen and six lots of oxygen. So we can work out the formula mass by multiplying each of these numbers by the right atomic mass and summing them together. So 24 for magnesium, 28 for nitrogen, and 96 for oxygen, giving us a total of 148. So our final answer is D, 148.
You might see this calculation done using the masses with atomic mass units. When talking about formula masses or atomic masses, we can sometimes leave these out just to simplify the calculation. But if we were being strict, we would include them.