A chemist performed a titration experiment in order to determine the concentrati

Question

A chemist performed a titration experiment in order to determine the concentration of a given aqueous solution of HCl (an acid). She pipeted 20.00 mL of the HCl solution into an Erlenmeyer flask. She then added about 15 mL of deionized water and 3 drops of phenolphthalein indicator dye to the flask. She titrated this mixture with 0.07481 M NaOH (aq), and obtained a permanent, pale pink endpoint after the addition of 24.21 mL of NaOH (aq). Given that the density of the HCl solution was 1.094 g/mL, determine the concentration of the original HCl acid solution in units of

a) molarity

b) percent by mass

Can you explain please?

Explanation / Answer

V1 = 20 ml

let Mi = M

so moles of HCl = 20*M

final volume = 20+15 = 35 ml

so concentration Mf = 20*M/35

V2 = 24.21 ml

M2 = 0.07481 M

Mf*V1 = M2*V2

20*M = 24.21*0.07481

M = 0.0905M.........................ANS

this is moles/volume so volume given is 20ml so mass of solution = density*volume = 1.094*20 = 21.88 gm

so mass of HCl = moles * molar mass = 0.020*0.0905 = 0.00181 moles

mass = 0.00181*36.5 = 0.066 gm

so mass % = 0.066/21.88 = 0.3 %..................ANS

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