Acid/Base Titration

Thurston High School Chemistry

Introduction

When acids and bases are added together a neutralization reaction takes place. An acid and a base combine to produce water and salt. For example:

nitric acid + potassium hydroxide water + salt

HNO3 + KOH HOH + KNO3

The acid donates a proton (H+) to the hydroxide (OH-) of the base to form water.

Q-1 Complete the following reaction. Have your instructor check your answer

Hydrochloric acid + sodium hydroxide water + salt

HCl + NaOH

To do a titration we use a buret

Q-2 What is the unit of volume on a buret?

Q-3 What is the maximum volume that can be measured with our burets?

Using the burets

• Rinse your buret with distilled water

• Gently pull the tip off and rinse it out also.

• The buret with the red dot must be used for acids and the buret with the blue dot must be used for bases.

• Fill your burets to approximately 30 mL. Be sure there are no air bubbles in the tip of your buret. Record your beginning volumes on your paper.

• To read the meniscus you must be at eye level, and read at the bottom of the dip.

• Holding a white card with a black line behind the buret can be very helpful

•Each line on our buret represents 1/10 of a mL so be sure to estimate your readings to .01 mL

Q-4 What is the reading here? Have your instructor check your answer.

• To release liquid turn, the stopcock. Be careful to not pull out on the stopcock as you turn it.

Q-5 Use your buret to measure out exactly 4.50 mL of acid into a clean Erlenmeyer Flask. Have your instructor check your results

Q-6 In your answer to Q-1 the mole ratio of acid to base is ____ : _____

In a titration with this ratio we can use the following relationship

Ma x Va = Mb x Vb

molarity of acid x volume of acid = molarity of base x volume of base

Q-7 Write this formula on your paper

In a titration we know the concentration of one of the solutions. This is known as the standard solution. We also can measure the volumes of acid and base using the burets. Once we know 3 of the variables we can calculate the concentration of the unknown acid or base.

Q-8 If 22.15 mL of 0.5 M NaOH is added to 16.50 mL of HCl, what is the concentration of the acid?

Ma x Va = Mb x Vb

Ex. If 15.20 mL of 1.0 M HCl is added to 30.60 mL of NaOH, what is the concentration of the base?

1.OM x 15.20 mL = Mb x 30.60 mL0.49 M = Mb

• The indicator phenolphthalein is used to tell when the acid and base exactly neutralize each other. This is called the endpoint of the titration.

• When phenolphthalein is added to an acid it remains clear. When it is added to a base it turns pink

Q-9 If your flask is clear you need to add more ______.

Q-10 If your flask is pink you need to add more ______.

• As liquid is added to the Erlenmeyer flask always swirl gently

• The endpoint of a titration is shown by a light pink color that stays for at least 30 seconds

• We start a titration by adding 10 mL of distilled water and 3 drops of phenolphthalein to a clean Erlenmeyer flask. Do this now.

• Q-11 Record the beginning volumes of acid and base. Also record the molarity of the acid.

• Add approximately 10 ml of acid to the flask and begin to add base.

• The first time through a titration we add the base quickly to turn the solution pink. You can then a small amount of acid and then titrate to the light pink endpoint.

• When you are close to the endpoint you often need to add very small amounts of acid or base. To do this, turn the stopcock just enough to have 1 small drop of liquid hanging from the bottom of the buret.

• Then touch the buret to the side of the flask

• Then use your squirt bottle to wash the drop in while you continue swirling the flask.

Q-12 When you reach the light pink end-point, record the final volume of acid and base. Show your instructor the end point and have your paper initialed.

• Q-13 Calculate the molarity of the base using the formula Ma x Va = Mb x Vb

• The key to a successful titration is patience!

• Always use clean equipment• Remember to record your beginning and ending volumes and read to .01 mL• Read the meniscus correctly• Use the formula Ma x Va = Mb x Vb for a 1:1 mole ratio to calculate the concentration of your unknown solution

Good Luck