How do you calculate the molar masses for Strontium Phosphate and Chromium(III)Nitride using significant figures correctly?
I am looking these atomic masses up at this site:
http://environmentalchemistry.com/yogi/reference/molar.html
For Sr3(PO4)2
Sr = 87.62 x 3 = 262.86
P = 30.97376 x 2 = 61.94752
O = 15.9994 x 8 = 127.9952
Total = 452.80272
The first multiplication allows 4 s.f. and I like to add one to keep from making rounding error; therefore, we will call it 262.86 (with the last place questionable).
The P has 7 s.f. so all will go.
The O has 6 s.f. so that will be 127.9952 with the 2 questionable.
Now, for adding the least precise number is .86 (two places to the right of the decimal); therefore, we can't have more than two places to the right of the decimal in the sum. That makes it 452.80 and since the 6 in .86 was carried as an "extra" I would truncate that last zero to make it 452.8
To calculate the molar masses of compounds using significant figures correctly, you need to determine the atomic masses of each element in the compound and then sum them up according to the molecular formula. Here's how you can do it for Strontium Phosphate (Sr3(PO4)2) and Chromium(III) Nitride (CrN):
1. Strontium Phosphate (Sr3(PO4)2):
- Strontium (Sr) has an atomic mass of 87.62 g/mol.
- Phosphorus (P) has an atomic mass of 30.97 g/mol.
- Oxygen (O) has an atomic mass of 15.999 g/mol.
To calculate the molar mass of Strontium Phosphate, you multiply the atomic masses by the number of atoms present in the compound. In this case, there are three strontium atoms and two phosphate groups, each consisting of one phosphorus atom and four oxygen atoms.
Molar mass of Sr3(PO4)2 = (3 * 87.62 g/mol) + (2 * (1 * 30.97 g/mol + 4 * 15.999 g/mol))
= 261.86 g/mol + (2 * (30.97 g/mol + 63.996 g/mol))
= 261.86 g/mol + (2 * 94.966 g/mol)
= 261.86 g/mol + 189.932 g/mol
= 451.792 g/mol
Therefore, the molar mass of Strontium Phosphate is 451.792 g/mol.
2. Chromium(III) Nitride (CrN):
- Chromium (Cr) has an atomic mass of 51.996 g/mol.
- Nitrogen (N) has an atomic mass of 14.007 g/mol.
In this case, since the compound is Chromium(III) Nitride, it means there is one chromium atom and three nitrogen atoms.
Molar mass of CrN = 1 * 51.996 g/mol + 3 * 14.007 g/mol
= 51.996 g/mol + 42.021 g/mol
= 94.017 g/mol
Therefore, the molar mass of Chromium(III) Nitride is 94.017 g/mol.
When calculating molar masses with significant figures, you should ensure that the atomic masses used for the calculation are reported with the appropriate number of significant figures.