Question

using the data colleced for part B on your data sheet, calculate the molar mass of the unknown acid. Show Calculations for trial #1 and the average for part B.

Part B Data:

Trial #1 Trial #2 Trial #3   

Mass unknown acid, g: 0.252g 0.250g 0.251g

Final buret reading, mL: 39.1mL 41.5mL 39.1mL

initial buret reading, mL: 2.90mL 2.70mL 2.90mL

volume NaOH used, mL: 36.2mL 38.8mL 36.2 mL

If someone could explain how I'm supposed to figure out the molar mass if i dont know what the formula of the acid to add up the grams is. I was able to do trial #1 for part A of this experiment but I can't do the same things because i am limited on information.

Thanks!

Answer 1

This exercise has two mistakes:

First mistake: We don't know if it's a monoprotic acid, diprotic acid or polyprotic acid. Example: HCl, H₂SO₄ or H₃PO₄.

Second mistake: We don't know the concentration of NaOH. We can assume, for example 0.1M, so:

mmol NaOH 1st determination: 0.1 mol/1000mL x 36.2 mL = 0.00362 mol= 3.62 mmolNaOH

mmol NaOH 2nd determination: 0.1 mol/1000mL x 38.8 mL = 0.00388 mol= 3.88 mmolNaOH

mmol NaOH 3rd determination: 0.1 mol/1000mL x 36.2 mL = 0.00362 mol= 3.62 mmolNaOH

But you need to know what kind of acid you have; if it's monoprotic, diprotic or polyprotic acid and you can write the equation.

Note: Dear student, please resend the question with this information.

Note: Dear student, if you know it's a monoprotic acid, you can write the following equation:

HA + NaOH NaA + H₂O

and you must calculate moles of NaOH, but you need to know the concentration of NaOH solution, we can assume 0.1M and then:

mmol NaOH 1st determination: 0.1 mol/1000mL x 36.2 mL = 0.00362 mol= 3.62 mmol NaOH 3.62 mmol HA

mmol NaOH 2nd determination: 0.1 mol/1000mL x 38.8 mL = 0.00388 mol= 3.88 mmol NaOH 3.88 mmol HA

mmol NaOH 3rd determination: 0.1 mol/1000mL x 36.2 mL = 0.00362 mol= 3.62 mmol NaOH 3.62 mmol HA

so,

1st determination: 252 mg HA / 3.62 mmol HA = 69.61 mg/mmol

2nd determination: 250 mg HA / 3.88 mmol HA = 64.43 mg/mmol

3rd determination: 251 mg HA / 3.62 mmol HA = 69.34 mg/mmol

But, remember you need to know the concentration of NaOH solution, and you can calculate molecular weight of the acid. Good Luck and Success!