[LYF]

can you check my solutions again, please....dun no why my teacher marked it wrong! Thanks so much!

Calculate the mass of the following. Avogadro's number = 6.022E23 particles/mol.

1) nitrogen in 7.14 mol (NH4)3PO4

Solution: (NH4)3(PO4) = 133 g/mol
Three nitrogens = 42
Proportion of nitrogen = 42/133 = 31.6%
7.14 mol x 0.316 = 2.25 mol of nitrogen (N)
2.25 mol x 14 g/mol = 31.6 grams of N
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2) oxygen in 5.52E+22 molecules SO3

Solution: n(O):n(SO3)=3:1 hence n(O)= 5.52E+22*3= 1.656E+23 so 1.656 mol
m(O)= M(O)*n(O)= 16* 1.656= 26.496 g

[enahs]

Let's start with the first one.

You start with 7.14 mols of ammonium phosphate. When we split everything apart into it's separate atoms, how do you end up with less mols of Nitrogen?

So to answer the question, you can do it two ways. Break all the atoms apart and count how many you have, that is, 3 * 7.14 mols of Nitrogen = 21.42 mols Nitrogen = 300 g Nitrogen.

Or, you can do percents like you did, but you used the molecular weight, not the total mass you have. You started with 7.14 mols, 7.14 * 133 g/mol = 950g , 950 g * 0.316 = 300g.


The second one.

1.646 * 10²³ molecules of oxygen is correct. But then why do you go from that to 1.656 mols? There are 6.022*10²³ molecules in a mol. If you have only 1.646 * 10²³ molecules you are missing 4.37*10²³ of the molecules in a mol. You have what percentage of a mol? And how much does that percentage of a mol weigh?



Slow down and think about your math. You do not need to be able to do the multiplication/division in your head to 5 decimal places to look at it and see if itchanged in the right direction. Ask your self after you do your math work, do I have more or less then I started with, should I have more or less? Did this change in the right direction? Based of of my chemical knowledge does this number make sense?

[LYF]

 :)thanks so much for your help, i got it now!