a 5.55g sample of a weak acid with ka=1.3*10^-4 was combined with 5.00ml of 6.00 M NAOH and the resulting solution was diluted to 750mL. The measured pH of the solution was 4.25. what is the molor mass of the weak acid.
if used the formula
pH=kpa+log(base/acid)
Ph=3.89+log(6.00/x)
(x)=2.62
i don't know how to find the molar mass
See your post above.
To find the molar mass of the weak acid, we need to use the given information and apply the concept of stoichiometry.
Given:
Mass of the weak acid = 5.55 g
Ka of the weak acid = 1.3 * 10^-4
Volume of NaOH solution = 5.00 mL
Concentration of NaOH = 6.00 M
Measured pH = 4.25
Dilution factor = 750 mL
To solve for the molar mass, we can follow these steps:
Step 1: Calculate the moles of NaOH used.
Moles of NaOH = concentration (in moles/L) * volume (in L)
Moles of NaOH = 6.00 mol/L * 0.005 L
Moles of NaOH = 0.03 mol
Step 2: Calculate the moles of the weak acid reacted with NaOH.
Since NaOH and the weak acid react in a 1:1 stoichiometric ratio, the moles of the weak acid are also 0.03 mol.
Step 3: Calculate the initial concentration of the weak acid.
Concentration of the weak acid = moles of weak acid / volume of solution (in L)
Concentration of the weak acid = 0.03 mol / 0.75 L
Concentration of the weak acid = 0.04 mol/L
Step 4: Calculate the initial concentration of the conjugate base (NaA).
Using the Henderson-Hasselbalch equation:
pH = pKa + log10 ([A-] / [HA])
Rearrange the equation to solve for [A-]:
[A-] / [HA] = 10^(pH - pKa)
[A-] / [HA] = 10^(4.25 - 3.89)
[A-] / [HA] = 1.414
Since we assumed the weak acid and the conjugate base concentration are equal initially:
[HA] = [A-]
Hence, [A-] / [HA] = 1
So, [A-] = [HA] = 0.04 mol/L
Step 5: Calculate the molar mass of the weak acid.
Molar mass = mass / moles
Molar mass = 5.55 g / 0.03 mol
Molar mass = 185 g/mol
Therefore, the molar mass of the weak acid is 185 g/mol.