Question

A solution of NaOH (aq) of unknown concentration, is added from a buret into an Erlenmeyer flask containing 25.00 ml of 0.2000 M HCI (aq). A small amount of phenolphthalein indicator is also present in the acid, initally. A very pale pink but permanent color appears in the Erlenmeyer upon the addition of 41.22 ml of the base solution. The appearance of the color signifies the end point of the titration.

a) Please write a balanced molecular equation for the titration reaction (2 points)

b) Please propose which of the three aqueous solution reaction types is occuring here (1 point)

c) Please calculate the concentration of the base solution (3 points)

d) Please determine how many moles of HCl are present in the Erlenmeyer initally (1 point)

e) If the first addition of base solution amounts to 5.20 mL please calculate how many moles of base are present in this amount

f) Please compare the moles of base added in the first addition to the moles of acid initally present. How many moles of base would be required to attain the end point? Please conclude whoch reactant is the limiting reactant (2 points)

Answer 1

a) The balanced reaction will be

NaOH + HCl ------ NaCl + H2O

b)

It is an acid-base reaction since NaOH(base) is reacting with the HCl(acid)

c)

For end point

Number of moles of NaOH = Number of moles of HCl

Volume of NaOH * Molarity of NaOH = Volume of HCl * Molarity of HCl

41.22 * Molarity of NaOH = 25 * 0.2000

Molarity of NaOH = 0.12130M

d)

Number of moles of HCl present = Volume of solution (in L) * Molarity of HCl (M)

=> 25/1000 * 0.2000

=> 0.005 moles

e)

Number of moles of base present = Volume of NaOH * Molarity of NaOH

=> 5.20/1000 * 0.12130

=> 0.00063076 moles

f) Number of moles of acid (0.005) is greater than number of moles of base (0.00063076 mole)

number of moies of base required = 0.0005 moles

Limiting reagent will be NaOH