A 75.0 mL volume of 0.200 M NH3 (Kb=1.8x10^-5) is titrated with 0.500 M HNO3. Calculate the pH after the addition of 28.0 mL of HNO3.
mmols NH3 = mL x M = 75.0 x 0.200 = 15.0
mmols HNO3 = 28.0 x 0.500 = 14.0
.......NH3 + HNO3 ==> NH4NO3
I......15.0....0........0
add...........14.0.......
C.....-14.0..-14.0.....+14.0
E.......1.0.....0......+14.0
So you are 1.0 mmol away from the equivalence point. Basically you have a buffer problem. You have a small amount of NH3 left and you have (relatively) large amount NH4^+ formed. Use the Henderson-Hasselbalch equation. NH3 will be the base and NH4^+ will be the acid.
Thanks.
To calculate the pH after the addition of 28.0 mL of HNO3, you will need to determine the moles and concentrations of the relevant species, then apply the appropriate equilibrium equation.
Here are the steps to follow:
Step 1: Calculate the moles of NH3 initially present.
Given that the volume of NH3 is 75.0 mL and the concentration is 0.200 M, you can calculate the moles of NH3:
Moles of NH3 = volume of NH3 (L) x concentration of NH3 (M)
= 75.0 mL ÷ 1000 mL/L x 0.200 M
Step 2: Calculate the moles of HNO3 added.
Given that the volume of HNO3 added is 28.0 mL and the concentration is 0.500 M, you can calculate the moles of HNO3:
Moles of HNO3 = volume of HNO3 (L) x concentration of HNO3 (M)
= 28.0 mL ÷ 1000 mL/L x 0.500 M
Step 3: Determine the excess or limiting reactant.
Compare the moles of NH3 and HNO3 to determine the limiting reactant. Since NH3 and HNO3 react in a 1:1 ratio, the limiting reactant will be the one with fewer moles. It is important to note that any excess moles of HNO3 will be used to calculate the pH.
Step 4: Calculate the moles of HNO3 remaining.
Subtract the moles of HNO3 used in the reaction from the initial moles of HNO3 added to determine how much is remaining.
Step 5: Calculate the concentration of NH4+ and OH- ions.
Since NH3 is a weak base, it reacts with water to form NH4+ and OH- ions. Use the balanced equation and the initial concentration of NH3 to calculate the concentrations of NH4+ and OH- ions.
Step 6: Calculate the concentration of H3O+ ions.
Calculate the concentration of H3O+ ions using the Kw expression, which states that Kw = [H3O+][OH-]. Since OH- concentration is known from step 5, you can solve for [H3O+] and convert it to pH.
It is important to note that these steps provide a general approach to solving the problem. Intermediate steps might be required, and rounding significant figures must be considered.