10.00 mL sample of aqueous HCl requires 31.00 mL of 0.0900 M NaOH to reach the endpoint. What is the molar
concentration of HCl.

Molar concentration of HCl = 0.279 M

Explanation:

Given data:

Volume of NaOH = 31.00 mL ( 31/1000 = 0.031 L)

Molarity of NaOH = 0.0900 M

Volume of HCl = 10.00 mL (0.01 L)

Molar concentration of HCl = ?

Solution:

Chemical equation:

NaOH + HCl    →   NaCl + H₂O

Number of moles of NaOH:

Number of moles = Molarity × volume in L

Number of moles = 0.0900 mol/L  × 0.031 L

Number of moles = 0.00279 mol

For one mole of HCl one mole of NaOH is required.

Number of moles of HCl = 0.00279 mol

Molar concentration of HCl = 0.00279 mol / 0.01 L

Molar concentration of HCl = 0.279 M

your answer would be a. 12.5 kg