Question

A student is asked to determine the molarity of a strong base by titrating it with 0.250 M solution of H2SO4. The students is instructed to pipet a 20.0 mL portion of the strong base solution into a conical flask, to add two drops of an indicator that changes at pH = 7, and to dispense the standard H2SO4 solution from a buret until the solution undergoes a permanent color change. The initial buret reading is 5.00 mL and the final reading is 30.00 mL at the equivalence point.What is the [OH-] in the strong base solution?0.750 M0.313 M0.625 M0.375 M

Answer 1

Answer: The concentration of hydroxide ions in the strong base solution is 0.625 M

Explanation:

To calculate the molarity of solution, we use the equation:

$\text{Molarity of the solution}=\frac{\text{Moles of solute}\times 1000}{\text{Volume of solution (in mL)}}$        .......(1)

For sulfuric acid:

Molarity of solution = 0.250 M

Volume of solution = (30.00 - 5.00) mL = 25.00 mL

Putting values in equation 1, we get:

$0.250M=\frac{\text{Moles of sulfuric acid}\times 1000}{25.00}\\\\\text{Moles of sulfuric acid}=\frac{0.250\times 25.00}{1000}=0.00625moles$

1 mole of sulfuric acid produces 2 moles of hydrogen ions and 1 mole of sulfate ions

Moles of hydrogen ions = $(2\times 0.00625)=0.0125moles$

At pH = 7

Moles of hydrogen ions = Moles of hydroxide ions = 0.0125 moles

Now, calculating the hydroxide ions in the base solution by using equation 1, we get:

Moles of hydroxide ions = 0.0125 moles

Volume of base solution = 20.0 mL

Putting values in equation 1, we get:

$\text{Molarity of }OH^-\text{ ions}=\frac{0.0125mol\times 1000}{20.0}\\\\\text{Molarity of }OH^-\text{ ions}=0.625M$

Hence, the concentration of hydroxide ions in the strong base solution is 0.625 M