# chem moles titrations

Three students each prepare a standard solution by dissolving 10.6g of solid from different bottles labelled 'sodium carbonate' in exactly 1dm^3 of water. They use this standard solution in a titration to determine the exact concentration of a solution of sulfuric acid at approximately 0.1moldm^−3. They each use a pipette to measure out exactly 25.00cm^3 of the standard solution into a conical flask, they each use the same indicator and they each carry out their titrations with great care and accuracy.

The volumes of sulfuric acid solution that they each use are listed below. Only student A finds the correct concentration of the sulfuric acid. Student B is within 20, percent, 20% but student C is so far out that they know something is wrong. Student C asks for help and is reminded that some solids can contain water of crystallization. Student A uses anhydrous sodium carbonate, but what is x in the formula Na2CO3.xH2O (s) for students B and C?

Student A:23.75cm^3
Student B: 20.20cm^3
Student C: 8.80cm^3

PART 1: exact concentration of the sulfuric acid: 0.105 moldm^-3
PART 2: find x for students B and C.

I am stuck on part 2. What equation do I use???
Thanks
Find the values you have and what you need to work out then tag me and I may be able to give a step by step (as these moles equations differ between exam boards)
Original post by Sasuto
Three students each prepare a standard solution by dissolving 10.6g of solid from different bottles labelled 'sodium carbonate' in exactly 1dm^3 of water. They use this standard solution in a titration to determine the exact concentration of a solution of sulfuric acid at approximately 0.1moldm^−3. They each use a pipette to measure out exactly 25.00cm^3 of the standard solution into a conical flask, they each use the same indicator and they each carry out their titrations with great care and accuracy.

The volumes of sulfuric acid solution that they each use are listed below. Only student A finds the correct concentration of the sulfuric acid. Student B is within 20, percent, 20% but student C is so far out that they know something is wrong. Student C asks for help and is reminded that some solids can contain water of crystallization. Student A uses anhydrous sodium carbonate, but what is x in the formula Na2CO3.xH2O (s) for students B and C?

Student A:23.75cm^3
Student B: 20.20cm^3
Student C: 8.80cm^3

PART 1: exact concentration of the sulfuric acid: 0.105 moldm^-3
PART 2: find x for students B and C.

I am stuck on part 2. What equation do I use???
Thanks

Hi, I just tried this question myself.
What I did was use the concentration we worked out in part A of the questions, and I found the moles of the sulphuric acid using the volume from student B. As the ratio is 1:1 its the same number of moles for the sodium carbonate. Then I worked out how many grams was in 25cm3. Then I used n=m/mr to work out the overall Mr for the hydrated sodium carbonate, then subtracted the mr of just sodium carbonate. Then divided that by the mr of water.
I got an answer for student B but I don't see how to get an answer for student C as I got an Mr of the hydrated sodium carbonate that is smaller than the mr of anhydrous sodium carbonate. Let me know if you want me to post a picture of my workings (only if the answers I got are right though lol)

Spoiler

Original post by fluffycloud101
Hi, I just tried this question myself.
What I did was use the concentration we worked out in part A of the questions, and I found the moles of the sulphuric acid using the volume from student B. As the ratio is 1:1 its the same number of moles for the sodium carbonate. Then I worked out how many grams was in 25cm3. Then I used n=m/mr to work out the overall Mr for the hydrated sodium carbonate, then subtracted the mr of just sodium carbonate. Then divided that by the mr of water.
I got an answer for student B but I don't see how to get an answer for student C as I got an Mr of the hydrated sodium carbonate that is smaller than the mr of anhydrous sodium carbonate. Let me know if you want me to post a picture of my workings (only if the answers I got are right though lol)

Spoiler

Hi, thanks for your help. I checked and b value of x is 1. for student c it is 10: I did the exact same as student b but i used 8.80cm^3 instead of 20.20cm^3
can you explain why you are able to find the Mr of HYDRATED sodium carbonate when using the mole of just sodium carbonate? thank you