neutrilisation pH calculation. (1 Viewer)

[4lettersdown]

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]

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]

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.

 

[mitochondria]

For future reference.

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.

20 and 30 are actually ambiguous. The significant figures of numbers without a decimal point and has trailing zeros typically depends on the context. Since this is titration you would expect the error in volume to be roughly 0.02-0.04 mL. Usually for questions like this you should specify the number of significant figures used.

The only reason why the answer for this question should have 1 sig. fig. in the final answer is because 0.08 and/or 0.05 both have 1 sig. fig.

 

[adomad]

Significant Figures
this site talks about sig figs. also, pH is always given to one dp regardless of the sig figs in the question. its just a rule

 

[mitochondria]

also, pH is always given to one dp regardless of the sig figs in the question. its just a rule

Would you mind editing that part of your post out, please? I really don't think such rule exists except in the classroom of a second-grade chemistry teacher.

 

[adomad]

Would you mind editing that part of your post out, please? I really don't think such rule exists except in the classroom of a second-grade chemistry teacher.

a HSC marker said that is how they mark.