# Titration of unknown acid in lab, how to determine molar mass?

Hi,

## Homework Statement

It is in my chemistry lab course. I had 0.1g of an unknown solid that I diluted in 100ml of distilled water. Then, I took 25ml and titrated it with NaOH (Concentration = 0.0189M). To reach equivalent point (the point where concentration of NaOH is equal to the concentration of my unknown IN THE SOLUTION), it took 7.5ml of NaOH.

## Homework Equations

C1V1=C2V2
M=n/V , where n is number of moles, and V is volume of solution in liter.

## The Attempt at a Solution

Basically, I did C1V1=C2V2.
C1= 0.0189M V1=7.5ml C2=? V2=32.5ml (25ml + 7.5ml)
I then get C2=0.00436.

When I solve of n (in M=n/V) and then do grams/moles to get molar mass, I get a molar mass that is impossible, or waay off... So I'm not sure anymore if I am approching this correctly...Is my V2 correct? How about the V I take for M=n/V, is it 0.100L (initial volume at which I diluted the solid).

Thanks in advance for any help.

PS: This is my first post on this forum, and it seems really a great forum!

Hi,

## Homework Statement

It is in my chemistry lab course. I had 0.1g of an unknown solid that I diluted in 100ml of distilled water. Then, I took 25ml and titrated it with NaOH (Concentration = 0.0189M). To reach equivalent point (the point where concentration of NaOH is equal to the concentration of my unknown IN THE SOLUTION), it took 7.5ml of NaOH.

## Homework Equations

C1V1=C2V2
M=n/V , where n is number of moles, and V is volume of solution in liter.

## The Attempt at a Solution

Basically, I did C1V1=C2V2.
C1= 0.0189M V1=7.5ml C2=? V2=32.5ml (25ml + 7.5ml)
I then get C2=0.00436.

When I solve of n (in M=n/V) and then do grams/moles to get molar mass, I get a molar mass that is impossible, or waay off... So I'm not sure anymore if I am approching this correctly...Is my V2 correct? How about the V I take for M=n/V, is it 0.100L (initial volume at which I diluted the solid).

Thanks in advance for any help.

PS: This is my first post on this forum, and it seems really a great forum!

I don't think your V2 value is right, as you are only titrating 25 ml at the start. Also don't forget that the 25 ml is not the complete sample.

OK. If I put my V2 to 25ml, it gives an answer that makes a lot of sens.
However, I don't understand the logic behind it. I mean, the final volume of titration is NOT 25ml, but rather 25ml + 7.5 ml. The initial concnetration of NaOH that I have is for 7.5ml, but as we add that 7.5 to 25, it gets diluted, and the final volume would not be 25....

OK. If I put my V2 to 25ml, it gives an answer that makes a lot of sens.
However, I don't understand the logic behind it. I mean, the final volume of titration is NOT 25ml, but rather 25ml + 7.5 ml. The initial concnetration of NaOH that I have is for 7.5ml, but as we add that 7.5 to 25, it gets diluted, and the final volume would not be 25....