# Homework Help: Chemistry help

1. Feb 17, 2005

### gokugreene

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

2. Feb 17, 2005

### Yapper

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: Feb 17, 2005
3. Feb 17, 2005

### Gokul43201

Staff Emeritus
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 !

4. Feb 17, 2005

### Yapper

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?

5. Feb 17, 2005

### Gokul43201

Staff Emeritus
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.

6. Feb 18, 2005

### Yapper

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

7. Feb 18, 2005

### gokugreene

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)

8. Feb 18, 2005

### Gokul43201

Staff Emeritus
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 ?

9. Feb 18, 2005

### gokugreene

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?

10. Feb 18, 2005

### dextercioby

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.

11. Feb 18, 2005

### Gokul43201

Staff Emeritus
No, there is no need for molar mass here.

12. Feb 18, 2005

### dextercioby

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

Daniel.

13. Feb 18, 2005

### Gokul43201

Staff Emeritus
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.

14. Feb 18, 2005

### dextercioby

Didn't notice.It is indeed a simple multiplication by 2...

Daniel.