A 30.00-mL sample of weak acid is titrated with 0.0167 M NaOH. At the endpoint , it is found the 40.35 mL of titrant was used. What was the concentration of the weak acid ?

I assume this is a monoprotic acid.

moles NaOH = M x L
moles weak acid = mol NaOH
M acid = moles acid/L acid.

To find the concentration of the weak acid, we can use the concept of stoichiometry and the principle of equivalence in a titration.

1. Start by writing down the balanced chemical equation for the reaction between the weak acid and NaOH. Let's assume the weak acid is represented as HA:

HA + NaOH → NaA + H2O

2. Determine the mole ratio between the weak acid and NaOH. Based on the balanced equation, the mole ratio is 1:1. This means that for each mole of weak acid (HA) reacted, one mole of NaOH is required.

3. Convert the volume of NaOH used in the titration to moles. In this case, 40.35 mL of the titrant (NaOH) was used. To convert mL to L, divide by 1000:

40.35 mL ÷ 1000 = 0.04035 L

Now, use the molarity of NaOH (0.0167 M) to calculate the number of moles:

Moles of NaOH = molarity × volume in liters

Moles of NaOH = 0.0167 M × 0.04035 L = 0.000674 mol

4. Since the mole ratio between the weak acid and NaOH is 1:1, we can conclude that the number of moles of HA (weak acid) used in the titration is also 0.000674 mol.

5. Calculate the concentration of the weak acid (HA) by dividing the number of moles by the volume of the weak acid sample. In this case, the volume of the weak acid sample is 30.00 mL, which can be converted to liters by dividing by 1000:

Volume of HA sample = 30.00 mL ÷ 1000 = 0.03000 L

Concentration of HA = moles of HA ÷ volume of HA sample

Concentration of HA = 0.000674 mol ÷ 0.03000 L = 0.0225 M

Therefore, the concentration of the weak acid is 0.0225 M.