I have 3 questions in chemistry that are giving me trouble

Question

Compare the number of accessible microstates for one molecule of H2O(l) to that of one molecule of H2O(g) at 298 K.?

 

 

 

Determine the volume of 17.4 glacial acetic acid (CH3COOH) and the mass of NaCH3COO(s) that you would have to use to prepare 500mL of a 20 mM buffer at pH 5.2. (The total concentration of CH3COOH + CH3OO- should be 20 mM)

Suggest an alternate way to prepare the same buffer without using glacial acetic acid(explanation not calculation)

Answer 1

I think that the Molarity is 17.4 is it? So 17.4M glacial acetic acid.

 

Using the pH equation (Henderson-Hasselbalch) you have:

 

pH = pKa+log([base]/[acid])   ; Ka for acetic acid is 1.8x10^-5 so pKa = -logKa = 4.74 ; since pH = 5.2 and you solve the equation to get the ratio of [base]/[acid] then:

 

[base]/[acid] = 2.88

 

since [acid] + [base] = 20mM then ===> [base] = 2.88[acid] ===> [acid] + 2.88[acid] = 20mM

 

[acid] = 5.15 mM

 

[base] = 14.85 mM

 

Then since you have 500ml of the buffer it will make it M=mol/V or mol = M.V  so 

 

mmol of acid = 2577 mmol of Acetic acid

 

mmol of base = 7423 mmol of acetate ion

 

converting these mmol to grams through molecular mass (60.05 g/mol for Acetic acid and 82.03 g/mol for sodium acetate) you will need:

 

155g of Acetic Acid 

609 g of Sodium acetate

 

As for an alternate way of preparing this buffer what you would need is to make a solution of sodium acetate which will get a basic pH. If you neutralize it by adding a concentrated strong acid such as HCl or H2SO4 you will make the sodium acetate react with the H3O+ ions coming from these acids forming acetic acid. Once you have enough acetic acid then you will have yourself a buffer since you will have acetate ion and its conjugate acid the acetic acid which happens to be a weak acid. The amount of strong acid depends on the pH you want to get.