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## Main Question or Discussion Point

Hello everyone. I have a slight problem with my chemistry and was wondering if there is someone that could help me. Chemistry is not my strong point so please go easy. Thank you.

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The problem is that I have gotten mixed up with how to calculate moles. Now, I know that I should use the formula:

n= Mass (g)/ Mr

The problem is that this doesn’t seem to apply everywhere and I don’t know why. If I have 1.5g of Na2CO3 in 250cm^3 of solution. Then I know that,

n= 1.5g/ 106 = 0.014150943mol

So, there is 0.014mol of Na2CO3 in 250cm^3 of solution.

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Now, I did a question in my textbook where this method of calculating moles did not work.

“A solution of Na2CO3 contains 12.5g of anhydrous salt in 1000cm^3 of solution. When 25cm^3 of this solution was titrated with a solution of HCl using methyl orange indicator, 23.45cm^3 of the acid was required.”

Naturally I would then use the formula to work out the moles…

n= 12.5g/ 106 = 0.117924528mol

However, the book states that this gives the concentration, and to calculate the moles I will have to multiply it by the volume (i.e. 25cm^3).

What am I not understanding? Any help would be greatly appreciated, Thank you.:)

---------------------------------------------------------------------

The problem is that I have gotten mixed up with how to calculate moles. Now, I know that I should use the formula:

n= Mass (g)/ Mr

The problem is that this doesn’t seem to apply everywhere and I don’t know why. If I have 1.5g of Na2CO3 in 250cm^3 of solution. Then I know that,

n= 1.5g/ 106 = 0.014150943mol

So, there is 0.014mol of Na2CO3 in 250cm^3 of solution.

---------------------------------------------------------------------

Now, I did a question in my textbook where this method of calculating moles did not work.

“A solution of Na2CO3 contains 12.5g of anhydrous salt in 1000cm^3 of solution. When 25cm^3 of this solution was titrated with a solution of HCl using methyl orange indicator, 23.45cm^3 of the acid was required.”

Naturally I would then use the formula to work out the moles…

n= 12.5g/ 106 = 0.117924528mol

However, the book states that this gives the concentration, and to calculate the moles I will have to multiply it by the volume (i.e. 25cm^3).

What am I not understanding? Any help would be greatly appreciated, Thank you.:)

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