# Multiple choice-Q14 (1 Viewer)

• ### 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