buffers

SP - resource used in webinar

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memorise this slide �(i can’t help w this- it’s on you)

Choosing an indicator: the pH range needs to lie within the equivalence point (straight line)

so basically,

Strong base used = phenolphthalein

Strong acid used = methyl orange

Explain what is meant by the term buffer solution. (2)

• Solution which resists change in pH / maintains pH
• despite the addition of (small amounts of) acid/base (or dilution)

(and memorise this slide asw)

• What can an acidic buffer can be mixture of? (2)
1. weak acid and a respective salt�OR
2. excess weak acid and a strong base

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^very important for WEAK ACID + STRONG BASE

Note: if only weak acid is present (e.g. propanoic acid in a beaker):

What assumption about the Ka equation do you make in buffer calc?

• acid is undissociated

so

• [A^-] = [salt]
• [HA] = [acid]

^^^^VERY VERY IMPORTANT FOR BUFFERS

(just write it out if in doubt)

3 main types of buffer questions �(these are what ppl find hard)

1. pH of buffer: weak acid and a respective salt (-oate, e.g.)
2. pH of buffer: weak acid and a strong alkali
3. pH of NEW buffer after hydroxide or acid added

Once you can do these 3, buffer calcs are v easy

TYPE 1 PAST PAPER Q) �pH of buffer: weak acid + respective salt

1. Conc of acid
2. Conc of salt
3. Use Ka equation

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• Finally, [H+] into pH

TYPE 1 PAST PAPER Q) �pH of buffer: weak acid + respective salt

1. We don’t like mass - convert to moles: 0.25/122.0 = 2.0491 x 10³ mol (Mr of benzoic acid = 122)

2. Conc of acid (easy enough) = mol / total vol = 2.0491 x 10³ / 0.25 (volume) = 8.1967 x 10-3 mol dm^-3

4. (memorise: slide 3), just put numbers: 6.3 x 10^-5 x 0.008/0.0132

= 3.81 x 10^-5 M (mol dm-3)

5. We now have conc of H+ so we can work out pH (-log[H+]) = 4.41

3. Do same for the salt: 0.475/144 = 3.2986 x 10-3 moles, conc = Answer / 0.25 = 0.0132 mol dm-3

Another Type 1 question I found online:

If you can do this, you’re sorted for Type 1: move onto Type 2

MS for that

Total volume (120 + 75)

NOTE:

TYPE 2 PAST Q) weak acid + strong BASE

• A buffer solution is prepared by adding 50.0 mL of 0.10 M sodium hydroxide (NaOH), a strong base, to 100.0 mL of 0.10 M acetic acid (CH₃COOH), a weak acid. Assume the final solution has a volume of 150.0 mL. Given that the Ka​ of acetic acid is 1.8×10⁻⁵, calculate the pH of the resulting buffer solution.

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• 1. Initial moles of acid and base
• 2. Final moles of acid and salt (after reaction)
• 3. Final concentrations of acid and salt
• 4. Use Ka equation to find [H+], then pH

• Calculate Initial Moles:�Moles of NaOH: 0.050 L x 0.10 M = 0.0050 moles�Moles of CH3COOH: 0.100 L x 0.10 M = 0.0100 moles

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• Stoichiometry (basically look at the coefficients):

NaOH and CH3COOH react in a 1:1 molar ratio.

Since ALL of the NaOH will react, it will neutralize an equal amount of CH3COOH, �leaving 0.0050 moles of CH3COOH unreacted and �producing 0.0050 moles of CH3COO-

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(This is the most important bit to properly understand - the rest is basic calculations)

Slide 3:

Calculate the pH:

With 0.0050 moles of CH3COOH and 0.0050 moles of CH3COO- in a total volume of 150 mL, the concentrations are:

[CH3COOH] = 0.0050 moles / 0.150 L = 0.0333 M

[CH3COO-] = 0.0050 moles / 0.150 L = 0.0333 M

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Calculate [H+]:

Ka = [H+][CH3COO-] / [CH3COOH]

1.8 x 10^-5 = [H+] * 0.0333 / 0.0333

[H+] = 1.8 x 10^-5

pH = -log([H+]) = -log(1.8 x10^-5)

pH = 4.74

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(Note: because HA and A^- are equal here, pKa = pH)

Type 3: pH of NEW buffer after hydroxide or acid added (probs hardest question)

Ka = 1.74 x 10-5

1. Initial concentrations 🡪 initial moles of acid and salt
2. Final moles after equilibrium shift
3. Final moles 🡪 final concentrations
4. Use Ka equation to find [H+]

• 0.15 mol = acid (because 1000cm3 = 1dm3)
• 0.10 mol = salt

BECAUSE YOU’RE ADDING H+, equilibrium moves towards ACID

• acid: 0.15 + 0.01 = 0.16 mol
• salt: 0.10 – 0.01 = 0.09 mol

[H+] = Ka [HA]/[A^-] = 1.74 x 10-5 x 0.16/0.09 = 3.09 x 10-5

pH = 4.51

((memorise Slide 3))

All 3 types covered

• hopefully, better understanding of buffer questions and what techniques to use

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• I will send you these slides in case they’re useful
• if you do find them useful, distribute them

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• MEMORISE slides 2 and 3 it’ll help a lot tm