Need Help - MC question (1 Viewer)

[Midnight_Lotus]

I can't figure out how they got the answer for the following calculation question:

20ml of 0.08mol/L HCl is mixed with 30ml of 0.05 mol/L NaOH
What is the pH of the resultant solution?

A) 1.1
B) 2.7
C) 4.0
D) 7.0

I keep getting 4.0 however the answer is supposed to be 2.7?

 

[b3kh1t]

First find the number of moles of H+ ions and OH- ions released.

using n=CxV
n(HCl)= 0.020x0.08 = 1.6x10^-3 moles
since HCl ionises to form one H+ per molecule then 1.6x10^-3 moles are present

n(NaOH)= 0.03x0.05= 1.5x10^-3
also since NaOH produces one OH- ion per molecule then 1.5x10^-3 moles are present

now the 1.5x10^-3 moles of OH- will neutralise 1.5x10^-3 of the H+ ions leaving 1x10^-4 moles of H+ remaining.
to find the pH you must use the concentration of the H+ ions, therefore C= n/V = 1x10^-4/(0.020+0.030) = 2x10^-3 mol/L (do NOT forget to find the concentration of H+ ions, this is where you went wrong, try to be very careful)
therefore the pH = -log(2x10^-3) = 2.699, thus the answer is B

 

[Midnight_Lotus]

Thanks! After i got the initial answer i solved for concentration but must have typed it in wrong on my calculator as i got 5x10^-6 (got right number now though)

 

[nephh22]

first find the number of moles of h+ ions and oh- ions released.

Using n=cxv
n(hcl)= 0.020x0.08 = 1.6x10^-3 moles
since hcl ionises to form one h+ per molecule then 1.6x10^-3 moles are present

n(naoh)= 0.03x0.05= 1.5x10^-3
also since naoh produces one oh- ion per molecule then 1.5x10^-3 moles are present

now the 1.5x10^-3 moles of oh- will neutralise 1.5x10^-3 of the h+ ions leaving 1x10^-4 moles of h+ remaining.
To find the ph you must use the concentration of the h+ ions, therefore c= n/v = 1x10^-4/(0.020+0.030) = 2x10^-3 mol/l (do not forget to find the concentration of h+ ions, this is where you went wrong, try to be very careful)
therefore the ph = -log(2x10^-3) = 2.699, thus the answer is b

this ^^^

 

[b3kh1t]

Thanks! After i got the initial answer i solved for concentration but must have typed it in wrong on my calculator as i got 5x10^-6 (got right number now though)

Aiight cool and best of luck tomorrow