# Chemistry help

Hey could you guys tell me whether I am on the right track with this or not.

First Problem:
A tablespoon of sugar, $$C_1_2H_2_2O_1_1$$, weighs 2.85g.

molar mass of sugar = 352g
molar mass of oxygen = 16g

A.) How many moles of sugar are there in a tablespoon?
(2.85g of sugar/1)(1 mole/352g of sugar)

B.) How many grams of O are there in a tablespoon?
(2.85g of sugar/1)(1 mole/352g of sugar)(16g of Oxygen/1 mole)(11 atoms of oxygen)

Am I doing those right? My teacher never went over it in class and I can't find any examples on the net.

Second Problem:
Sample A is $$1.555~x~10^{-5}$$ mole of $$AlK (SO_4)_2~12H_2O$$
molar mass of $$AlK (SO_4)_2~12H_2O$$ = 462g
molar mass of oxygen = 16.00g

A.) How many grams of O are present?
($$1.555~x~10^{-5}$$/1)(462g/1 mole)(16g of oxygen/1 mole)(22 atoms of oxygen)

Is this how you do it?

Thanks

For the first problem I do not know why you multiplied by 11 atoms of oxygen, doesnt multipling by 16g O per mole already give u the grams of oxygen?

I didnt look at your second problem but you might want to check for the same problem

Last edited:
Gokul43201
Staff Emeritus
Gold Member
Everytihng looks good, except for a tiny error in the second question - there are 20 (not 22) atoms of O in the hydrated molecule.

Your method, otherwise, is perfect !

Gokul43201 isnt dimentional analysis multiplying by one and just changing the units? but he multiplied the 2.85 by 11, you would only do that if there where 11 tablespoons of sugar right?

Gokul43201
Staff Emeritus
Gold Member
No, he multiplied by 11 because there are 11 atoms of O per molecule of glucose.

Weight of O in tablespoon = wt. of O atom * # of O atoms in tablespoon

# of O atoms = 11 * # of glucose molecules in tablespoon

# of glucose molecules in tbsp = N(av) * # of moles of glucose

# of moles of glucose = (wt. of glucose) / (molar mass of glucose)

And wt. of O atom * N(av) = molar mass of O

Combining all of the above gives the formula that greene used.

Oh I see I was just thinking about it a different way

How would I calculate how many $$SO_4^{-2}$$ ions?
molar mass of $$(SO_4^{-2})_2$$ = 192.14g

Would you do this? (1.55 x 10^-5/1)(462g/1 mole)(192.14g/1 mole)(1 mole/6.022 x 10^23)

Gokul43201
Staff Emeritus
Gold Member
No, check the units. It doesn't match up.

How many sulfate ions per molecule of AlK(SO4)2 ?

How many molecules in a mole of anything ? So, in x moles ?

So it would be (1.55 x 10^-5/1)(462g/1 mole)(192.14g/1 mole)(6.022 x 10^23/1 mole) right?

dextercioby
Homework Helper
I can't follow your giant formula,but i'll give you a hint how to do it:
1 mol of $KAl(SO_{4})_{2}\cdot 12H_{2}O$--------------->320g O
$1.555\cdot 10^{-5}$ moles of $KAl(SO_{4})_{2}\cdot 12 H_{2}O$--------------->x g O

x=...?

Daniel.

Gokul43201
Staff Emeritus
Gold Member
gokugreene said:
So it would be (1.55 x 10^-5/1)(462g/1 mole)(192.14g/1 mole)(6.022 x 10^23/1 mole) right?
No, there is no need for molar mass here.

dextercioby
Homework Helper
Gokul,please don't tell me you're making him do some horrid calculations,when he could just do JUST a simple multiplication...

Daniel.

Gokul43201
Staff Emeritus
Gold Member
dextercioby said:
Gokul,please don't tell me you're making him do some horrid calculations,when he could just do JUST a simple multiplication...

Daniel.

It is a simple multiplication.

And he's not trying to find the number of grams of oxygen anymore. He has to calculate the number of sulphate ions.

dextercioby