You have prepared 20.00 mL of a buffer, using 4.00 mL 0.010 M HA (weak acid) and 3.00 mL 0.010 M NaA (provides A-, the conjugate base of HA). pKa for HA is 6.25. How many moles of HA will be present in the mixture after addition of 0.20 mL 0.010 M NaOH?
The buffer starts with 4 mL x 0.01 = 0.04 mllimols HA and
3 mL x 0.01 = 0.03 millimoles A^-
........HA + OH^= --> A^- + H2O
I......0.04..0.......0.03.....
add........0.002--------------
C....-0.002.-0.002.+0.002.............
E...0.038....0.....0.032..........
So you have 0.038 millimols of the acid. Change that to mols.
To determine the number of moles of HA present in the mixture after the addition of NaOH, we will follow these steps:
Step 1: Calculate the initial moles of HA and A-.
Step 2: Determine the moles of NaOH added.
Step 3: React NaOH with A- to form HA.
Step 4: Calculate the remaining moles of HA.
Let's tackle each step one by one:
Step 1: Calculate the initial moles of HA and A-.
a) Moles of HA:
We are given the volume of HA as 4.00 mL and the concentration of HA as 0.010 M. The formula to calculate moles is:
Moles = Volume (in L) × Concentration
Converting the volume from mL to L:
4.00 mL = 4.00 × 10^(-3) L
Now, calculating the moles of HA:
Moles of HA = (4.00 × 10^(-3) L) × (0.010 M)
b) Moles of A-:
We are given the volume of NaA as 3.00 mL and the concentration of NaA as 0.010 M. The formula to calculate moles is:
Moles = Volume (in L) × Concentration
Converting the volume from mL to L:
3.00 mL = 3.00 × 10^(-3) L
Now, calculating the moles of A-:
Moles of A- = (3.00 × 10^(-3) L) × (0.010 M)
Step 2: Determine the moles of NaOH added.
We are given the volume of NaOH added as 0.20 mL and the concentration of NaOH as 0.010 M. The formula to calculate moles is:
Moles = Volume (in L) × Concentration
Converting the volume from mL to L:
0.20 mL = 0.20 × 10^(-3) L
Now, calculating the moles of NaOH:
Moles of NaOH = (0.20 × 10^(-3) L) × (0.010 M)
Step 3: React NaOH with A- to form HA.
NaOH reacts with A- to form HA. The equation for this reaction is:
NaOH + A- -> HA + Na+
Since both NaOH and A- have a 1:1 stoichiometric ratio, the moles of NaOH reacted will be equal to the moles of HA formed.
Step 4: Calculate the remaining moles of HA.
To find the remaining moles of HA, we subtract the moles of HA formed in Step 3 from the initial moles of HA.
Remaining Moles of HA = Initial Moles of HA - Moles of NaOH (formed HA)
Now, you can substitute the values into the equation to calculate the remaining moles of HA.