Question

When 1.00 g of thiamine hydrochloride (also called vitamin B1 hydrochloride) is dissolved in water, then diluted to 10.00 mL, the pH of the resulting solution is 4.50. The formula weight of thiamine hydrochloride is 337.3 g/mol. Calculate Ka.

Answer 1

Answer:

Ka= 3.37×$10^{-9}$

Explanation:

Ka is the measure of an acid's ability to donate H+ ions. A strong acid will have a larger Ka value. Consider the following equation

HA + H2O → H3O+ + A-

where,

HA is acid, which when dissolves in a solution dissociates into hydronium ion and A- ion. Ka will be represented as

Ka = $\frac{[H+] [A-]}{[HA]}$

Step 1

calculate the number of moles of thiamine hydrochloride

no of moles(n)=g/MM

                    n=1.00/337.3

                   n=2.96×$10^{-3}$

Step 2

The solution was diluted to 10.00ml

∵ 2.96×$10^{-3}$/ 0.01 L

  =0.296M

Step 3

Find H+ using the formula

pH= -log [H+}

4.50= -log [H+]

[H]= $10^{-4.50}$

[H]=3.16×$10^{-5}$

Step 4

Substitude the above values in the formula

Ka=$\frac{[H+] [A-]}{[HA]}$

   =$\frac{[0.0000316] [0.000316]}{0.296}$

  = 3.37×$10^{-9}$