Question

What is the pH of 0.20 gram of H2CO3 in 3,500 mL

Answer 1

The pH is 2.74.

Step-by-step explanation:

1st) We have to calculate the moles of H2CO3 contained in 0.20g, using the molar mass of the acid (62g/mol):

`0.20g*(1mole)/(62g)=0.0032moles`

Now we know that there are 0.0032 moles of H2CO3.

2nd) It is necessary to calculate the molarity of the solution:

`\begin{gathered} 3,500mL-0.0032moles \\ 1,000mL-x=(1,000mL*0.0032moles)/(3,500mL) \\ x=9.14*10^(-4)moles \end{gathered}`

The molarity of the solution is 9.14*10^-4 M.

3rd) We have to calculate the concentration of H+.

Since H2CO3 is a strong acid, it completely dissociates in aqueous solution. This is the dissociation euqation:

`H_2CO_3\rightarrow2H^++CO_3^(-2)`

When H2CO3 dissociates, 2 protons (H+) are produced, that's why the [H+] it is twice the [H2CO3].

So, the concentration of H+ is 1.83*10^-3 M.

4th) Now we can calculate the pH of the solution, by replacing the [H+] in the pH formula:

`\begin{gathered} pH=-log\lbrack H^+\rbrack \\ pH=-log(1.83*10^(-3)) \\ pH=2.74 \end{gathered}`

So, the pH is 2.74.