neutrilisation pH calculation.

[4lettersdown]

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hey guys, im having a little trouble with this question:

20ml of 0.08mol/L HCl added to 30ml of 0.05mol/L of NaOH
find the pH of the resulting solution

[Fortify]

n (HCl) = C * V
n (HCl) = 0.08 * 0.02
n (HCl) = 1.6*10^-3mols

n (NaOH) = C * V
n (NaOH) = 0.05 * 0.03
n (NaOH) = 1.5*10^-3mols

n (excess) = n (HCl) - n (NaOH)
n (excess) = 1.6*10^-3 - 1.5*10^-3
n (excess) = 1*10^-4 mols of HCl

C = n / V
C = 1*10^-4 / 0.05
C = 2*10^-3mols

pH = - log [H+]
pH = - log [2*10^-3]
pH = 2.7

[4lettersdown]

thanks a lot
but..... amirite in saying it would actually be 3 because both 20 ml and 30 ml are only one significant figure?

[tizona]

its weird because according to the catholic trials the ph must be to the amount of decimal places, not sig figs. can anyone confirm that please?

[Fortify]

Originally Posted by 4lettersdown

thanks a lot
but..... amirite in saying it would actually be 3 because both 20 ml and 30 ml are only one significant figure?

That's interesting. I normally do pH to 1 or 2 decimal places :S

[4lettersdown]

uh oh

[nosh22]

For pH calculations, significant figures in the question must equal decimal places in the answer. (Weird, I know.)

[4lettersdown]

so its one decimal place?

[nosh22]

Yeah.

[Fortify]

2.70 then .

[4lettersdown]

faceplam.
2.7

[iMAN2]

Originally Posted by 4lettersdown

thanks a lot
but..... amirite in saying it would actually be 3 because both 20 ml and 30 ml are only one significant figure?

This is correct. As the lowest amount of significant figures in the question is 1 i.e. 20 your answer must be also to 1 significant figure. Thus the answer is 3.

In chemistry we work with significant figures and not decimal places for calculations (in most cases). Titration may vary if they specificy to how many decimal places but they usually specificy 'significant figures' anyway.