[1c2h3e4m]

i have to calcuate molar mass of stuff but i dont understand some of it, in the book it says to calculate Ba(NO3)2     (3 and 2 and subscript)   then u need to find the mass of each, Ba - 137, N- 14 and O - 16 about but then it says there are 6 mols in O and 2 mols in N, where did this come from? Ba has 1 it said but i dont understand where these mols came from? after that i just times 1 by 137 + 2 times 14 + 6 times 16 but i dont understand where the 1, 2 and 6 came from, help plz?

[enahs]

Those numbers come from the subscripts. The subscripts indicates how many you have.

In your case, Ba(NO₃)₂.
That means you have 1 Ba (If no subscript is there, the 1 is implied, it is just impractical to always write 1). If you have a subscript after a bracket, when you have that number of things in the bracket.
So you have
1 Ba
2 NO₃
But each NO₃ has 3 Oxygens. So 2 NO₃'s means you have 2 N's and 6 O's.


http://www.fordhamprep.org/gcurran/sho/sho/lessons/lesson53.htm

[1c2h3e4m]

so how would calcium phosphate be written, cause for  barium nitrate it is Ba(NO3)2, but barium is Ba2+ and nitrate is NO3-, how do you know when to put () and what number goes after it and stuff.

[enahs]

You pretty much just answered your own question.

You said:

but barium is Ba2+ and nitrate is NO3-

In order for the molecule to be neutral (no positive or negative charge) you have to balance the charges. If Ba has 2+, you need 2 -'s to cancel it and bring it back to 0 (neutral). If NO₃^- has only 1 negative charge, you need 2 of them to cancel the 2+ charge of Ba²⁺.

Now apply that concept to Calcium phosphate.

[1c2h3e4m]

could anyone write the formula for calcium phosphate cause if i see it, it would help me figure out where it came from.  Is it Ca(PO4)2???  if not please show me what it is.

[Sev]

Enahs gave good tips with his last post. 

Calcium is 2+, phosphate 3-, so there must be 3 Ca²⁺ for every 2 PO₄^3-: Ca₃(PO₄)₂

[1c2h3e4m]

so would there be 3 Ca  2 P   and 8 O???

[Sev]

Correct 

[1c2h3e4m]

ok if you want to could your see if this is correct


Molar mass for calcium phosphate = 310g/mol
Molar mass of TiO2 = 80g/mol

[Sev]

Correct 

Though I got 310.1 and 79.9 (difference due to rounding)

[1c2h3e4m]

yeah, thanks, what would ammonium sulfate be tho?  its NH4 +  SO4 2-     NH4 + already has a 4 as its subscript so would you like add 2 to it cause the SO4 2- has 2 or what would you do? im confused on this now.

[Sev]

It has to be (NH₄)₂SO₄ (to be neutral).  NH₄ is 1+, SO₄ is 2- so ratio is 2:1.