Hydrogen cyanide is a weak acid with Ka = 4.9 x 10-10. Calculate the pH of a solution (to 2 decimal places) containing 2.28 g of Sr(CN)2 per 158 mL.
Sr(CN)2 ==> Sr+^2 + 2CN^-
CN^- + HOH ==> HCN + OH^-
Kb = Kw/Ka = (HCN)(OH^-)/(CN^-)
Convert 2.28 g Sr(CN)2 per 0.158L to mols/L.(Note that Sr(CN)2 has 2 CN^- per mole of Sr(CN)2.)
Plug x for (HCN) and x for (OH^-), solve for (OH^-), convert to pOH, then to pH.
Post your work if you get stuck.
Above posted by DrBob222.
To calculate the pH of the solution, we need to determine the concentration of hydrogen cyanide (HCN) in the solution.
Step 1: Find the moles of Sr(CN)2 in the solution:
The molar mass of Sr(CN)2 is 232.71 g/mol. Therefore, the number of moles of Sr(CN)2 can be calculated using the formula:
moles = mass / molar mass
moles = 2.28 g / 232.71 g/mol
moles ≈ 0.00980 mol
Step 2: Calculate the concentration of HCN:
Since 1 mole of Sr(CN)2 produces 2 moles of HCN, the number of moles of HCN can be calculated as follows:
moles of HCN = 2 × moles of Sr(CN)2
moles of HCN = 2 × 0.00980 mol
moles of HCN ≈ 0.0196 mol
The volume of the solution is given as 158 mL, which can be converted to liters:
volume = 158 mL / 1000 mL/L
volume = 0.158 L
Now we can calculate the concentration of HCN:
concentration = moles of HCN / volume
concentration = 0.0196 mol / 0.158 L
concentration ≈ 0.124 M
Step 3: Calculate the pH using the Ka value:
The Ka of hydrogen cyanide (HCN) is given as 4.9 × 10^(-10). Since it is a weak acid, we can assume that the dissociation of HCN will be negligible compared to the initial concentration. Therefore, we can assume that the concentration of HCN is equal to the concentration of [H+].
pH = -log[H+]
pH = -log(0.124)
pH ≈ 0.91
Therefore, the pH of the solution is approximately 0.91.