Chemistry Help: Calculating Moles and Mass of Sugar and AlK (SO4)2 12H2O

[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

 

[Yapper]

For the first problem I do not know why you multiplied by 11 atoms of oxygen, doesn't 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

 

[Gokul43201]

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 !

 

[Yapper]

Gokul43201 isn't dimensional 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]

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.

 

[Yapper]

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

 

[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)

 

[Gokul43201]

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 ?

 

[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?

 

[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.

 

[Gokul43201]

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.

Just answer each of my above questions...the answer will follow from there.

 

[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.

 

[Gokul43201]

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]

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

Daniel.