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İHSAN DOĞRAMACI FOUNDATION

BİLKENT ERZURUM LABORATORY SCHOOL

2nd Semester

2011-2012 Academic Year

9th Grade Chemistry

LAB REPORT 2

Name: Aysuda Pasinlioğlu

(Partner: İrem Küçük)

Topic: Observing Acids and Bases by Using Litmus Paper

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Observing Acids and Bases by Using Litmus Paper

Purpose of the Experiment: To learn how to use litmus paper to determine

the pH of the acids and bases.

Hypothesis: pH degrees of each substance will vary when litmus paper is used.

Introduction: In chemistry, pH is a measure of the acidity or basicity of an aqueous

solution by discriminate the colors. In the picture below, you can see how it is decided and

examples.

Another way to test acids and bases is using titration. An acid-base TITRATION is the

determination of the concentration of an acid or base by exactly neutralizing the acid/base

with an acid or base of known concentration.

ACID-BASE TITRATIONS

Molarities of acidic and basic solutions are often used to convert back and forth between

moles of solutes and volumes of their solutions, but how were the molarities of these solutions

determined? This section describes a procedure called titration, which can be used to find the

molarity of a solution of an acid or a base.

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In titration, one solution (solution #1) is added to another solution (solution #2) until a

chemical reaction between the components in the solutions has run to completion. Solution #1

is called the titrant, and we say that it is used to titrate solution #2. The completion of reaction

is usually shown by a change of color caused by a substance called an indicator.

A typical titration proceeds in the following way. A specific volume of the solution to be

titrated (solution #2) is poured into an Erlenmeyer flask (Figure 1). For example, 25.00 mL of

a nitric acid solution of unknown concentration might be added to a 250 mL Erlenmeyer

flask.

A solution of a substance that reacts with the solute in solution #2 is added to a buret. (A

buret is a laboratory instrument used to add measured volumes of solutions to other

containers.) This solution in the buret, which has a known concentration, is the titrant. The

buret is set up over the Erlenmeyer flask so the titrant can be added in a

controlled manner to the solution to be titrated (Figure 1). For example, a 0.115

M NaOH solution might be added to a buret, which is set

up over the Erlenmeyer flask containing the nitric acid solution.

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Figure 1    Setup for a Typical Titration     In a typical titration, the titrant in the buret is

added to the solution in the Erlenmeyer flask until the indicator changes color to show that

the reaction is complete.

An indicator is added to the solution being titrated. The indicator is a substance that changes

color when the reaction is complete. In our example, phenolphthalein, which is a commonly

used acid-base indicator, is added to the nitric acid solution in the Erlenmeyer flask.

Phenolphthalein has two chemical forms. In acidic conditions, it is in the acid form, which is

colorless. In basic conditions, an H+ ion is removed from each phenolphthalein molecule,

converting it to its base form, which is red.

The titrant is slowly added to the solution being titrated until the indicator changes color,

showing that the reaction is complete. This stage in the procedure is called the endpoint. In

our example, the NaOH solution is slowly added from the buret until the mixture in the

Erlenmeyer flask changes from colorless to red. The OH- ions in the NaOH solution react with

the H3O+ ions in the HNO3 solution.

H3O+(aq)  +  OH-(aq)    H2O(l)

As long as there are excess H3O+ ions in the solution, the solution stays acidic, the

phenolphthalein stays mostly in the acid form, and the solution is colorless. When enough

NaOH solution is added to react with all of the H3O+ions, the reaction is complete. When a

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small amount of extra NaOH solution is added, perhaps one drop, there will be an excess of

hydroxide ions, OH-, in solution. These react with the phenolphthalein molecules, changing

them from the acid form to the base form. Because the base form is red, the solution turns red,

telling us that the reaction is complete (or just slightly beyond complete).

The volume of titrant added from the buret is measured. For our example, lets assume that

18.3 mL of 0.115 M NaOH has been added. The following setup shows how the molarity of

the nitric acid solution can be calculated from this data.

 

 

 

                     =    or  0.0842 M HNO3

The first step the dimensional analysis thought-process is to clearly identify the units that you

want. Molarity describes the number of moles of solute per liter of solution, so we start by

placing moles of HNO3 over 1 L HNO3 solution.

Because molarity is a ratio of two units, we begin our calculation with a ratio of two units.

Knowing that we want volume of HNO3 solution on the bottom when we are done, we place

25.00 mL HNO3 solution on the bottom at the start. We place 18.3 mL NaOH solution on the

top of our ratio, giving us the ratio of two units overall that we want.

We convert milliliters of HNO3 solution to liters of HNO3 solution using the relationship

between milliliters and liters to convert milliliters. The last two conversion factors convert

from amount of one substance in a chemical reaction (mL NaOH solution) to amount of

another substance in the reaction (mol HNO3). Thus, this is an equation stoichiometry

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problem that requires at its core the conversion of moles of NaOH to moles of HNO3 using

the molar ratio for the reaction between them.

NaOH(aq)  +  HNO3(aq)    NaNO3(aq)  +  H2O(l)

In order to use the molar ratio to convert from moles of NaOH to moles of HNO3, we need to

convert from volume of NaOH solution to moles of NaOH using the molarity as a conversion

factor.

A GOOD WEBSITE TO LEARN ABOUT TITRATION (ALSO I USED )

http://www.mpcfaculty.net/mark_bishop/titration.htm

Examples of

titration are given in the textbook on the pages 128 and 129.

Example 1: 25 cm3 of vinegar were neutralized by 20 cm3 of 1 M sodium hydroxide

solution. What is the concentration of the ethanoic acid in the vinegar?

Step 1 calculate the number of moles of sodium hydroxide used.

1000 cm3 of 1 M solution contains 1 mole so 20 cm3 contains 20

100x1 mole or 0.02 mole.

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Step 2 from the equation; find the molar ratio of acid to alkali.

CH3COOH (aq) + NaOH (aq) CH3COONa (aq) + H2O (l)

1 mole 1 mole

The ratio is 1 mole of acid to 1 mole of alkali.

Step 3 work out the number of moles of acid neutralized.

1 mole of alkali neutralizes 1 mole of acid so

0.02 mole of alkali neutralizes 0.02 mole of acid.

Step 4 calculate the concentration of the acid. (25 cm3 = 0.025 dm3)

concentration= number of moles

volume∈dm3¿

= 0.020.025

¿ = 0.8 mol/dm3

So the concentration of etanoic acid in the vinegar is 0.8 mole.

Research Question:How does alkalines and acids change when using litmus paper? How

can you understand whether it’s a base or an acid?

Introduction:

We use pH scale to determine whether a material is acid or a base.Ph scales has some values

on it.The values are between 0-14.Point 7 shows the neutrality.It is the pH of neutral

substances. If the pH value of material is higher then 7 it is called base and when value gets

closer to 14 the basic quantities increases.If the value is smaller than 7 it is called acid and

when value gets closer to 0 the acidic quantities increases.

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Litmus paper is an indicator which uses to understand if the material is an acid or a base and it

has different colours when tested with materials.We are supposed to find out whether a

material is an acid or a base by looking at the change in litmus paper.

Setup:

Before we start to work on the experiment, laboratory, equipments and safety should be

double-checked.

List of Equipment

*Litmus Paper *Carbonate * Hydrocloric Acid

*Apple

Vinegar *Beaker

*Sulphuric

Acid

*Lemon *Ammonia * Lemon vinegar

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*Water *Sodium Hydrox

In order to reach more accurate results, the experiment should be

repeated many times (with more variables.) For each experiment, make sure that all items are

ready to be used.

Procedure to Follow:

First of all, provide the apparatus required for the experiment.

Fill the beaker with the material you are working on.

Make sure it weights as exactly same in following procedure.

Submerge the litmus paper into the beaker

Observe change in color on the litmus paper

Compare the results with the scale on your litmus paper.

Reach a conclusion by comparing the colors.

Find out if it is an acid or a base.

After the time, change the material you are working and repeat the same steps by only

changing the variable.

Repeat these processes as the number of material you will test.

Lastly, use your datas to get find out the change that is made by an acid or a base.

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Datas:

First Experiment:

Material: HCl (Hydrochloric Acid)

Change in color: Dark red (purple)

pH:1

Result: Strong Acid

The materials which have a Ph value smaller than 7 are called acids and as

you get closer to the 0, the acidic quantities increase. So it is strong acid because of having

more acid quantities.

Second Experiment:

Material: NaOH (Sodium Hydroxide)

Change in color: Dark Blue

pH: 14

Result: Strong Base

Sodium Hydroxide in a strong base which turned the litmus into dark blue

and its Ph is 14.

Third Experiment:

Material: NH3 (ammonia)

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Change in Color: Blue

pH: Between 10-12(approximately 11)

Result: Base

Fourth Experiment:

Material: Na2CO3 (sodium carbonate)

Change in Color: Green (light blue)

pH:11

Result: Base

As the bases change the litmus paper’s color in to green-blue or dark blue

, we can find out the fact that carbonate is a base which we can see from the scale.

Fifth experiment:

Material: H2SO4 (Sulphuric Acid)

Change in Color: Purple

pH:1

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Result: Strong Acid

Sixth experiment:

Material: Lemon

Change in Color: Reddish

pH:2

Result: Acidic

Seventh Experiment:

Material: Apple Vinegar

Change in Color: Dark Yellow

pH:3

Result: Acidic

Eighth Experiment:

Material: Limon Vinegar

Change in Color: Dark Yellow (reddish)

pH:2

Result: Acidic

Ninth Experiment:

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Material: Water

Change in Color: Very Light Green

pH:7

Result: Nearly neutral

!!! According to the experiments we have been working on , litmus paper changes its color

according to the pH level of substances as below:

For acids it turns into red brownish and purple as it’s smaller than pH of 7.

For bases it turns into blue-dark blue as it’s higher than pH of 7.

Finally, for neutral substances it remains colorless of light green or yellow its pH level is

7.

Conclusion of the Experiment:

Finally, as we have found out the fact that different level of pH has different color

changes on litmus paper. As we know some substances have different levels of pH. We

observe those substances into three groups which are acids, bases and neutral substances.

While we are working on the experiment, pH levels of the variables vary and we reached

different results. So our hypothesis became true. We have observed that the substances which

have pH value smaller than 7 actually mean acid and turn the litmus paper into red – purple

colors. Bases which are higher than 7 as their pH level turn the litmus paper into blue and

dark blue. Last group of pH scale which is neutral materials have pH value of 7 and turn the

litmus paper into light green or colorless. Also, we make sure that the amount of substances

we were working with were measured accurately to reach some exact values.

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