Question

After we transfer a 100 ml sample of the HCl solution into a 125 ml erlenmeyer flask, we add 50 ml of distilled water and 3 drops of phenolphthalein indicator solution. The resulting solution requires 44 mL of 0.125M NaOH solution to reach the end point.

Calculate the number of Moles NaOH needed., and the Molarity of HCl.

Please help... I somehow got a 284 Molarity solution...

Answer 1

Lets assume the molarity of HCl solution to be x M

so, 100 ml of this solution will contain x M*100/1000 moles of HCl (moles = molarity*volume in L)

= x/10 moles

If we dilute this 100ml solution by adding 50 ml of water the new molarity of HCl = moles/new volume

new volume = 100ml + 50ml = 150ml = 150/1000 L = 0.15 L

so, new molarity = (x/10 moles)/0.15 L = 2x/3 M

but the moles of HCl is still x/10 moles

Now, we add NaOH to reach the end point

At the end point the moles of NaOH = moles of HCl = x/10 moles

Moles of NaOH in 44ml of 0.125 M NaOH = 0.125M*44/1000 L (moles = molarity*volume in L)

= 0.0055 moles

So, x/10 moles = 0.0055 moles

so, x = 0.055 M

So, molartiy of sample HCl solution = x = 0.055 M

Moles of NaOH needed = 0.0055 moles