Chemical Forums

Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: NTP on March 19, 2007, 11:58:41 AM

Title: Moles
Post by: NTP on March 19, 2007, 11:58:41 AM

Hi,

Hi,

CAN SOMEONE EXPLAIN THE REASONING BEHIND THIS SUM, THANK YOU.

Write down the mass of calcium which has the same number of atoms as 12g of magnesium.



Not sure if I worked it right, I first worked out the number of atoms of 12gmagnesium and the result was 3 *10 23. Then I used this and to find the mass of calcium by doing moles of atoms= mass / ram, and my final answer was 1.2 *10 25

wOULD APPRECIATE A FEEDBACK , THANK YOU
Title: Re: Moles
Post by: AWK on March 19, 2007, 12:10:09 PM
12 g of Mg is equivalent to 1/2 mole. How many grams weights 1/2 mole of Ca?
Title: Re: Moles
Post by: NTP on March 19, 2007, 12:16:12 PM
20g is 1/2 of Ca
Title: Re: Moles
Post by: xiankai on March 20, 2007, 04:25:58 AM
and there is your correct answer
Title: Re: Moles
Post by: NTP on March 20, 2007, 07:28:02 AM
But if the elements are from different groups not like it was with the previous answer both group 2 elements, it has to be worked different?
Example:

Write down the mass of silver which has the same number of atoms as 3g of aluminium.

I worked it out the same way and my answer is 7.2 * 1024g of Ag. Not sure if it correct.
Title: Re: Moles
Post by: AWK on March 20, 2007, 11:50:09 AM
3 g of Al is approx. 1/9 mole - take 1/9 mole of Ag
Title: Re: Moles
Post by: DevaDevil on March 20, 2007, 06:19:32 PM
Quote from: NTP on March 20, 2007, 07:28:02 AM
Write down the mass of silver which has the same number of atoms as 3g of aluminium.

I worked it out the same way and my answer is 7.2 * 1024g of Ag.

O.o, that would mean that 3g aluminium has the same amount of moles as 7,200,000,000,000,000,000 tonnes of silver? :o

Somehow I don't think that's correct ;)



You do not need Avogadro's number for these calculations. Just calculate mass to moles (mass [g] / molar mass [g/mol] = moles, do not calculate to molecules) and then back to mass of the other element.
As a mol of a compound has the same number of molecules independant of the compound, moles are easier to work with than molecules. That is precisely why the unit was introduced :)
Title: Re: Moles
Post by: AWK on March 21, 2007, 02:11:09 AM
Quote
I worked it out the same way and my answer is 7.2 * 1024g of Ag. Not sure if it correct.
It does not matter 10-24 or 1024, isn't it?
Title: Re: Moles
Post by: NTP on March 21, 2007, 06:50:53 AM

I worked it out this way using moles so it is correct now hehe  :)

Number of moles  = mass / ram    (3g of Al)

N = 3/ 27
N= 0.1111111

Therefore: number of moles =  mass /ram   (mass of Ag).
                  0.1111111  =    mass / 108
                0.1111111 * 108
 Mass of Ag = 12g

My Answer is 12g of Ag .
Still not sure but hehe
Title: Re: Moles
Post by: xiankai on March 21, 2007, 08:02:18 AM
right on the dot again you are. its quite easy and merely involves simple arithmetic.
Title: Re: Moles
Post by: NTP on March 21, 2007, 02:29:27 PM

Thank you all for the help  ;)