Multiple choice-Q14 (1 Viewer)

Answer to Q14 multi

• d)

• Total voters
133

Member
What did people get..
20mL of 0.008 Mol/L HCl is mixed with 30 mL of 0.05 Mol/L NaOH.
What is the resultant PH.

a) 1.1
b) 2.7
c) 4.0
d) 7.0

how i did it--->

n(H+ in HCl)= 0.02 x 0.08 = 1.6x10^-3

n(OH- in NaOH)= 0.03 x 0.05 = 1.5x10^-3
1.6x10^-3 - 1.5x10^-3 = 0.1x10^-3 (H+ ions left inreacted)
-log(0.1x10^-3) = 4
so i got c).... evry 1 else saying other things though.

Undermyskin

Self-delusive
You forgot to find the CONCENTRATION of H+. You just found the mole and then fell into the trap.

It's B

Aaron.Judd

Member
For 14 I got C, Ph of 4.

Member
Shitttttttttttttttttttttttttttttttt!!!!!!

-::Sanni::-

Member
I got c) as well.... XP Wait...what's the correct way to work it out then? Aaron.Judd

Member
I worked it out doing all the calculations. Its in excess remember!!!

danz90

Active Member
Undermyskin said:
You forgot to find the CONCENTRATION of H+. You just found the mole and then fell into the trap.

It's B
Exactly, thats a common trap people fall into.

I agree in saying it's B.

-::Sanni::-

Member
Yeah, I worked it out in exactly the same way as cookiemonsterz's first post. But Undermyskin said that b) is the answer. How did they get b)... I can't see it! XP

I'm sorry I'm stupid...

vds700

Member
i did the calculations and got 1 mol HCl leftover, found concentration using c = n/V, subbed into pH formula, but kept getting an answer that wasnt one of the optoions, so just gave up and put b), hope thats right.

iEdd

Member
It's B. Find the number of H+ ions remaining and divide by 50mL. Then -log it. Answer is 2.69 or something ~= 2.7 (B).

I still say C.

iEdd

Member
Show your working then. If you did what the OP did, you are wrong. If you disagree with the fact that you need to find CONCENTRATION, not number of H+ ions, then you are also wrong. @who

Member
damn. i put d. 7 . lol i tried the calculations and came out with a weird answer so i just said since they are both strong acid / base then it must neutralise . fark me

fishbulb

Member
The answer is B. HCL is in excess by 0.0001 moles.
Therefore molarity = 0.0001/0.05 and pH = 2.7

Kearnzo

Member
n(H+ in HCl)= 0.02 x 0.08 = 1.6x10^-3

n(OH- in NaOH)= 0.03 x 0.05 = 1.5x10^-3

That is right

But 0.0016 - 0.0015 is 0.0001

This is number of moles of H+ ions left in the solutions

Ph = -log10[H+]

c = n/v

volume is 20mls +30 mls = 0.05 L

therefore [H+] is 0.0001/0.05 = 0.002

Ph = -log10[0.002] is equal to 2.69... = 2.7

Edmund1234

New Member
i did all that workin and put c wtf. maybe i put it wrong into calculator farrrrrk

Last edited:

Muchi

Member
na.. its b man, i forgot like an idiot to divide the excess hcl by its volume before logging the answer! pft, but i guess its okay coz i got 12/15 according to my teacher (who after test saw what answers i circled, i circled on sheet and booklet)

Aaron.Judd

Member
My chem teacher (JRAHS) says it is C.

al02

New Member
yeah, i got b too

mick135

Member
lol i put d
i thought - strong acid - strong base...
must be d
FARRRQ