Molecular weight of compound question

psu_213, BSN, RN · Jan 31, 2012

Part of the issue is that molecular weights are going to differ (in very small amounts) from source to source. The weight that is listed on periodic tables is an 'average' weight of all the isotopes of that element based on the average amount of each isotope found. In both example your answers of off such a small amount from the 'correct' answers that minute differences in the weights given and round off error could be contributing to the variations in answers. For example, you used an atomic weight of oxygen as 16.00. Most sources I say listed an atomic weight/mass of oxygen at 15.9994. Very, very small difference, but a difference none the less.

If there is confusion ask your instructor why your answers are different from theirs/the books. Unless they can come up with a better reason that "this is what the book says" then your answers are fine.

P.S. the questions may be worded as "molecular weight" but they probably mean "molecular mass" since molecular weight can vary depending on one's location (i.e. the weight would be different at sea level vs. at the top of Mt. Everest...it is dependent on gravity) while mass would be the same at basically all locations in the universe--but that is probably beyond the scope of the class.

2bEsqtoRN · Jan 31, 2012

The molecular weight of compounds? I'm not sure if it's my calculator or what but there are two answers that I get but my prof gets something different.
1.) Ca3N2
Ca 3 x 40.18= 120.54
N 2 x 14.01= 28.02
=
I get 148.56 each time BUT the answers say 148.26
2.) Li3PO4
Li 3 x 6.94= 20.82
P 1 x 30.07= 30.07
O 4 x 16.00= 64
=
I get 114.89 each time BUT the answers say 115.793.
HELP!!!
Thanks in advance! :)

I think you have the atomic weight of Calcium (Ca) and Phosphorous (P) wrong. According to the list of elements, Ca's weight is 40.078 and P, 30.97. Where did you get your periodic table from? Be careful and make sure you're using the correct list. :)