what is the pH of 0.80 M NaCN? What is the concentration of HCN in the solution?
The CN^- is hydrolyzed.
...........CN^- + HOH ==> HCN + OH^-
I.........0.8..............0.....0
C..........-x..............x.....x
E.........0.8-x............x.....x
Kb for CN^- = (Kw/Ka for HCN) = (x)(x)/(0.8-x) and solve for x = (OH^-) = (HCN)
This give you the HCN. Convert OH^- to pH.
To find the pH of a solution of NaCN, we need to consider the dissociation of NaCN in water. NaCN dissociates to form Na+ ions and CN- ions. CN- ions can react with water to form HCN and OH- ions through a reversible reaction.
Here is the step-by-step process to find the pH of the solution:
Step 1: Write the balanced dissociation reaction for NaCN.
NaCN (aq) → Na+ (aq) + CN- (aq)
Step 2: Calculate the concentration of CN- ions formed by NaCN. Since NaCN fully dissociates, the concentration of CN- ions is the same as the concentration of NaCN.
Given that the concentration of NaCN is 0.80 M, the concentration of CN- ions is also 0.80 M.
Step 3: Calculate the concentration of HCN. Since CN- reacts with water to form HCN, the concentration of HCN can be calculated using the equilibrium constant expression, Ka.
The equilibrium constant expression for the reaction is:
Ka = [HCN] / [CN-]
Given that the Ka for HCN is 4.9 x 10^-10, and the concentration of CN- ions is 0.80 M, we can use the equation:
4.9 x 10^-10 = [HCN] / 0.8
Rearranging the equation to solve for [HCN]:
[HCN] = 4.9 x 10^-10 * 0.8
Step 4: Calculate the pH using the concentration of HCN.
To find the pH, we need to determine the concentration of hydrogen ions (H+ ions). Since HCN is a weak acid, it partially dissociates to form H+ ions and CN- ions. However, the dissociation of HCN is small, so we can assume that the concentration of H+ ions is equal to the concentration of HCN.
Using the concentration of HCN calculated in Step 3, the concentration of H+ ions in the solution is also 4.9 x 10^-10 * 0.8.
Finally, to find the pH, we use the equation:
pH = -log[H+]
Substituting the concentration of H+ ions into the equation, we get:
pH = -log(4.9 x 10^-10 * 0.8)
After calculating this expression, we will obtain the pH of the solution.
To find the pH of a solution, we need to determine if the compound ionizes or dissociates in water. In the case of NaCN, it is a salt that dissociates into sodium ions (Na⁺) and cyanide ions (CN⁻) when dissolved in water. However, the cyanide ion (CN⁻) is a weak base that reacts with water to form hydrocyanic acid (HCN), which is a weak acid.
To determine the pH of the solution, we need to consider the dissociation of HCN. The pKa value of HCN is necessary to calculate the concentration of HCN and, consequently, the pH of the solution.
The pKa value of HCN is approximately 9.2. You might find this value in a chemistry textbook, handbook, or through an online search.
Given the concentration of NaCN is 0.80 M, this tells us the concentration of CN⁻ ions in the solution. To determine the concentration of HCN, we need to use the equation for the dissociation of HCN:
HCN ⇌ H⁺ + CN⁻
The equilibrium constant for this reaction is known as the acid dissociation constant (Ka), and we can use the pKa value to calculate it. The equation relating pKa and Ka is:
pKa = -log(Ka)
Solving for Ka:
Ka = 10^(-pKa)
Given pKa = 9.2, we can calculate Ka:
Ka = 10^(-9.2)
Now, let x be the concentration of HCN in mol/L. Since it is a weak acid, we can assume that the change in concentration of CN⁻ due to the dissociation of HCN will be negligible compared to the initial concentration of CN⁻.
Therefore, the concentration of CN⁻ remaining in solution will be approximately 0.80 M. The concentration of HCN formed by this dissociation will be x.
Using the equation for the dissociation of HCN, we can write an expression for the equilibrium constant:
Ka = [H⁺][CN⁻] / [HCN]
Substituting the values:
10^(-9.2) = [H⁺][0.80] / x
Rearranging the equation:
x = [H⁺] = [0.80 × 10^(-9.2)] / 10^(-0.80)
x = [0.80 × 10^(-9.2)] / 10^(-0.80)
Simplifying:
x ≈ 4.93 × 10^(-10)
Therefore, the concentration of HCN in the solution is approximately 4.93 × 10^(-10) M.
To find the pH, we need to calculate the concentration of H⁺ ions, which is the same as the concentration of HCN since it is a weak acid. The pH is defined as the negative logarithm (base 10) of the concentration of H⁺ ions:
pH = -log[H⁺]
pH ≈ -log[4.93 × 10^(-10)]
Calculating this value will give you the pH of the solution.