The material covered in this homework assignment applies to processes that occur

Question

The material covered in this homework assignment applies to processes that occur at the Earth's surface. Equilibrium constants that you may need are given. Show all your work. A solution comprising 1557.4 mg L^-1 of HCO_3^- has a pOH of 4.9. Calculate the total concentration of carbonic acid in the solution. Determine whether water comprising 5 times 10^-3 mol L^-1 of Ca^2+ and 2 times 10^-3 mol L^-1 of F^- is saturated with respect to fluorite. If the above water is supersaturated, calculate the number of g L^-1 of fluorite that should precipitate when equilibrium is reached. Calculate the concentrations of all ions and the pH of a solution containing 0.001 moles of silicic acid (H_4SiO_4j) per liter of solution. Silicic acid is a weak acid so you can treat it as a monoprotic acid. If the 0.001 moles silicic acid was added to a liter solution that had a pH of 4.2, what percentage of the silicic acid would dissociate?

Explanation / Answer

(1) pOH= 4.9

pH= 14-4.9 = 9.1

Given conc of HCO3 - is 1557.4 mg/L

molar mass of HCO3- is 61 g/mol

So, concn of HCO3- = 1.557 g/L / 61 g/mol =0.0255 M

The HCO3- reaction is

HCO3- H+ + CO32- with K2= 10-10.3

The CO32- generated on the past reaction will be the one that generate the H2CO3 in the solution, as it follows

H2CO3    H+ + HCO3-   with K1=10-6.35

For calculating the [HCO3-] you uses the K2 equation:

K2 = ([H+] * [CO32-]) / [HCO3-] from pH you can calculate [H+], and from stoichiometry you know that [H+] =[CO32-]

if pH = -Log [H+] => [H+] = 10-9.1 => [H+] = 7.94. Now you can calculate [HCO3-]

[HCO3-] = ([H+] * [CO32-]) / K2 = 1.25 x 10-8

[HCO3-] = 1.25 x 10-8

Now that you knows [HCO3-] you put this information on the K1 equation (notice that [HCO3-] = [H+])

k1= [H2CO3] / [H+] * [HCO3-] =>  [H2CO3] = [H+] * [HCO3-] * k1 = 7.07 x 10-23

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