# Thread: Major help needed with molecular masses homework.

1. Hi, I'm new so please be nice but im desperately in need of help!
Right, I'm not actually thick lol, but this first section of the AS course is killing me after just the 1st week!

1)How many moles of sugar in 1kG of sugar? *c12-h22-o11* Well I got (1000/342g) = 2.92 moles, not sure if its right.

Now how do I work out the number of molecules of sugar in 1Kg?
*if the relative molec mass is 342*

2)How many moles of water *18* in 90g of water? - I got (90/18 ) = 5
Then it asks, how many molecules is that? - This is where I get quite confused, could somebody please explain?
It also asks, how many atoms are present in that amount? - I tried 5xAvogadro's constant. Is this how it is done?

3)The mass of one molecule of a compound is 7.33 x 10^ -23. Calculate its relative molecular mass. *Confuseddd.com*

4)Calculate the mass of 50 litres of methane (ch4) at STP.
No clue and it's doing my head in.

I have just started a new sixth form at a selective school and have been placed in the top chemistry set based on my gcse grade, reference and aptitude test score. Sadly I feel well out of my depth so if somebody could please explain this to me, it would really help me regain my confidence and would be much appreciated!

2.

3. Originally Posted by Aerox217
Hi, I'm new so please be nice but im desperately in need of help!
Right, I'm not actually thick lol, but this first section of the AS course is killing me after just the 1st week!

1)How many moles of sugar in 1kG of sugar? *c12-h22-o11* Well I got (1000/342g) = 2.92 moles, not sure if its right.
Yup, looks about right.

Originally Posted by Aerox217
Now how do I work out the number of molecules of sugar in 1Kg?
*if the relative molec mass is 342*
Avogadro's constant, , is the number of particles in a mole of that particular substance. So to work out the number of molecules from the number of moles, simply times by .

Originally Posted by Aerox217
2)How many moles of water *18* in 90g of water? - I got (90/18 ) = 5
Then it asks, how many molecules is that? - This is where I get quite confused, could somebody please explain?
It also asks, how many atoms are present in that amount? - I tried 5xAvogadro's constant. Is this how it is done?
Yes, it is. I wrote about it above, because I didn't read the whole post thoroughly before responding

Originally Posted by Aerox217
3)The mass of one molecule of a compound is 7.33 x 10^ -23. Calculate its relative molecular mass. *Confuseddd.com*
try timesing by avogardo's constant to find the mass of one mole of the substance; this should be the same as the RAM.

Originally Posted by Aerox217
4)Calculate the mass of 50 litres of methane (ch4) at STP.
No clue and it's doing my head in.
At the 'chemistry version' of STP (standard temperature and pressure) one mole of any gas occupies 24 litres. So:

Originally Posted by Aerox217
I have just started a new sixth form at a selective school and have been placed in the top chemistry set based on my gcse grade, reference and aptitude test score. Sadly I feel well out of my depth so if somebody could please explain this to me, it would really help me regain my confidence and would be much appreciated!
No worries mate, I'm currently doing A2 chemistry, and spent a fair amount of last year helping people in similar positions to yours. I have been at the same selective Grammar school from Y7 to A2, so I never really got set any work I couldn't do.

4. Ahh cheers so much, I get most of that.
Only thing that still aint clear is:
you said Moles x Avogadro's constant = Number of molecules which I understand now
but what about finding the number of atoms present in that 90g of water?
The rest is crystal clear thanks.
Your help is much appreciated :-D

5. Originally Posted by Aerox217
but what about finding the number of atoms present in that 90g of water?
number of molecules times the number of atoms per molecule

6. One correction to make, drowsy. One mole of gas at STP occupies 22.4 liters, not 24.

It was a pleasure having you post here, Aerox217. I wish every student put effort into their work before asking questions like you did. Feel free to ask more questions if you need help.

7. Ahh, a bit shocked I didn't get that atom part lol, well obvious. You guys are great and I really appreciate your help. I always make sure I think it through first before I ask for help so np!
Oh and yeah our chemistry teacher has been telling us STP is 24dm^3, so I got confused seeing 22.4L all over the internet, think I'll use 22.4

8. Originally Posted by Chemboy
One correction to make, drowsy. One mole of gas at STP occupies 22.4 liters, not 24.
It depends what you call STP. I happen to know that for GCSE/A level chemistry, STP is 293K and 1 atmosphere pressure, at which one mole of gas occupies 24 litres. You're right though, STP is usually 273K, and one mole occupies 22.4 litres, but for his exams he will (most likely) need to use 24.

9. ok, I didn't realize that. That really annoys me...there should not be more than one "standard" temperature and pressure.

10. Yeah, me too. Especially when I use a different one in physics to chemistry...

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