A2 Chem Help: Acids and Bases Question

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i'mlaura
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I'm on a-levelchemistry.co.uk at the moment doing Unit 4 Acid and Base questions.

How do you know if it is an acid or base?
Bronsted lowry acid- H+ donor
Bronsted lowry base- H+ acceptor

1. State whether the following species can behave as an acid, as a base, or both:

a) NH4+ (A: Base)
b) NH3 (A: Acid)
c) H3O+ (A: Acid and Base)
d) HClO4 (A: Acid)
e) CO32- (A: Acid and Base)
f) NO3- (A: Base)
g) CH3CH2OH (A: Acid and Base)
h) CH3COOH (A: Acid and Base)
i) HSO4- (A: Acid and Base)
j) HNO3 (A: Acid and Base)
k) HCl (A: Acid and Base)

This is meant to be an easy question. Thanks!
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Pigster
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a) A
b) B
c) A
d) A
e) B
f) B
g) both
h) both
i) both
j) both
k) A

I think.
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i'mlaura
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(Original post by Pigster)
a) A
b) B
c) A
d) A
e) B
f) B
g) both
h) both
i) both
j) both
k) A

I think.
Thank you, that's correct for most of them. How did you know that though?
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i'mlaura
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(Original post by Pigster)
a) A
b) B
c) A
d) A
e) B
f) B
g) both
h) both
i) both
j) both
k) A

I think.

Also...

Do you know how you would work out this question?

Calculate the pH of 0.002 mol dm-3 of KOH using the equation pH= -lg[H+]

The answer is 11.3 but I got 2.70
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Farseer
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(Original post by i'mlaura)
Also...

Do you know how you would work out this question?

Calculate the pH of 0.002 mol dm-3 of KOH using the equation pH= -lg[H+]

The answer is 11.3 but I got 2.70
KOH is an alkali it gives out OH- not H+

you need Kw to work out this question
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i'mlaura
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(Original post by Farseer)
KOH is an alkali it gives out OH- not H+

you need Kw to work out this question
Ohhh thanks, I understand now!
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Pigster
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(Original post by i'mlaura)
Thank you, that's correct for most of them. How did you know that though?
I have been called a chemistry teacher, occasionally.

a) NH4+ (A: Base) NH4+ can donate an H+ so acts as an acid
b) NH3 (A: Acid) NH3 has a lone pair, so can accept - base
c) H3O+ (A: Acid and Base) H3O+ has a lone pair, so could accept another H+, but is nearly impossible
d) HClO4 (A: Acid) H2ClO4+ can exist, but wouldn't like it either
e) CO32- (A: Acid and Base) CO32- doesn't contain H, so can't donate H+, so can't be an acid
f) NO3- (A: Base) correct
g) CH3CH2OH (A: Acid and Base) ethanol can accept protons, but isn't especially happy about it. Does the reaction between ethanol and Na involve H+ donation? is it an acid?
h) CH3COOH (A: Acid and Base) ethanoic acid is deffo an acid, but can be protonated by stronger acids
i) HSO4- (A: Acid and Base) correct
j) HNO3 (A: Acid and Base) nitric is usually an acid, but can be protonated by stronger acids e.g. nitration of benzene
k) HCl (A: Acid and Base) hydrochloric is deffo an acid, H2Cl+ can form, but like H4O2+ isn't too happy about it (OK slightly less grumpy)

You don't have to use Kw to work out the pH of strong bases:

pOH = -log[OH-] = -log 0.002 = 2.70

pH = 14.00 - pOH = 14.00 - 2.70 = 11.30
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Borek
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(Original post by Pigster)
You don't have to use Kw to work out the pH of strong bases:

pOH = -log[OH-] = -log 0.002 = 2.70

pH = 14.00 - pOH = 14.00 - 2.70 = 11.30
Technically you just did:

pH + pOH = pKw

and 14 that you used to convert pOH to pH is nothing else but pKw.
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i'mlaura
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Thank you!!

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