# Concentration question 2007 past paper HELP PLEASE # 27 (1 Viewer)

#### HKHSCstudent

##### Member
2007 past paper #27(a)

If the volume of the water sample being tested is 50mL and the mass of the dired ppt obtained is 3.65 g, calculate the chlorine ion concentration in the water sample in ppm.

Is this right?

3.65g/50.0 mL=3.65g/(0.05L)=73g/L

73g/L X1000= 73000mg/L=73000ppm

1ppm=1mg/L=1g/L x1000

EDIT: mistake

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#### Undermyskin

##### Self-delusive
What is 'dired ppt'?

I think you have to divide 3.65 by the molar mass of that 'dired ppt' first, calculate the mass of chloride ion (not chlorine, mind you) on its own. Then follow your above process with this mass of chloride ion.

73000 ppm seems a bit too high for me.

EDIT: hang on, is it AgCl?
EDIT2: 3.65/(107.9+35.45) = A, A* 35.45= B (g); B/0.05=... g/L = ... ppm

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#### jessi90

##### Member
obviously he/she meant dried ppt...

nope, OP seems right to me

#### HKHSCstudent

##### Member

Thanks for help yeah dried precipitate , do you know where to get past paper solutions? 73000 is really too big

#### HKHSCstudent

##### Member

Undermyskin said:
What is 'dired ppt'?

I think you have to divide 3.65 by the molar mass of that 'dired ppt' first, calculate the mass of chloride ion (not chlorine, mind you) on its own. Then follow your above process with this mass of chloride ion.

73000 ppm seems a bit too high for me.

EDIT: hang on, is it AgCl?
EDIT2: 3.65/(107.9+35.45) = A, A* 35.45= B (g); B/0.05=... g/L = ... ppm
Why is it AgCl?Do I have to assume it to be Ag Cl because it is an ion?

#### minijumbuk

##### ┗(^o^ )┓三
Re: 回覆: Re: Concentration question 2007 past paper HELP PLEASE # 27

Oh shit. I didn't read the WHOLE question before. There is no way you can do this question without knowing the whole question.

Yes, the precipitate must be AgCl. You do not assume it is AgCl. You deduce it.

AgNO3 + Cl- --> AgCl + NO3-

So yeah, what Undermyskin did was certainly correct.

#### HKHSCstudent

##### Member

thanks, Good Luck in the Trial exams to everyone