Maharashtra Board Class 11 Chemistry Chapter 2 Introduction to analytical chemistry PDF Download

Introduction To Analytical Chemistry

2.1 Introduction

Analytical chemistry helps us study the chemical composition of substances. It uses instruments and methods to separate, identify and quantify matter under study. The analysis gives us chemical or physical information about a sample. Analysis may be qualitative or quantitative. Qualitative analysis finds out if elements or compounds are present or absent. Quantitative analysis finds out how much of each element or compound is present.

Teacher's Note

Analytical chemistry is used to check if milk is pure or if there are harmful chemicals in water. In India, food safety officers use these methods to check if food is safe to eat.

Exam Trick

Remember: Qualitative = Yes or No (is it there or not). Quantitative = How much (the amount). Just like when you check if sugar is in tea (qualitative) or how many spoons of sugar are in tea (quantitative).

Points to Remember

Analytical chemistry studies separation and identification of substances.
Qualitative analysis tells us if something is present or absent.
Quantitative analysis tells us the amount of something present.
Both methods use special tools and techniques.
Chemical analysis is important in medicine, farming and industry.

Importance Of Analytical Chemistry

Analytical chemistry helps us understand general, inorganic and organic chemistry better. Chemical analysis is very important to check the composition of raw materials, products and air in factories. In farming, it checks soil and fertilizer composition. In medicine, it checks medicine composition. Analytical chemistry is used in forensic science, engineering and industry. It helps in making new materials. Analytical chemistry has both classical wet chemical methods and modern instrumental methods.

2.2 Analysis

Analysis is done on a small sample of material, not on the entire bulk. When the sample is a few grams, it is called semi-microanalysis. It has two types: qualitative and quantitative.

Classical qualitative analysis methods use separations such as precipitation, extraction and distillation. Identification may be based on differences in colour, odour, melting point, boiling point and reactivity. Classical quantitative methods use volumetric analysis and gravimetric analysis.

2.2.1 Chemical Methods Of Qualitative Analysis

Chemical analysis is done mainly in two stages. The dry method does not dissolve the sample. The wet method dissolves the sample first and then analyzes it. The dry method is usually used as preliminary tests in qualitative analysis.

Semi-micro qualitative analysis uses apparatus such as test tubes, beakers, evaporating dish, crucible, spot plate, watch glass, wire gauze, water bath, burner, blowpipe, pair of tongs and centrifuge.

Qualitative analysis of organic and inorganic compounds involves different types of tests. Most organic compounds have a small number of elements. The most important ones are carbon, hydrogen, oxygen, nitrogen, sulphur, halogen and phosphorous. Elementary qualitative analysis finds out if these elements are present. Identification of an organic compound involves tests such as detection of functional group and finding melting and boiling point. Qualitative analysis of simple inorganic compounds involves detection and confirmation of cationic and anionic species in them.

Teacher's Note

Qualitative analysis is like a doctor's simple blood test that shows if you have an infection or not. Quantitative analysis is like measuring exactly how much sugar is in your blood.

Exam Trick

Remember: Dry method = no water added. Wet method = water added. Think of it like testing soil - dry soil test is faster, wet soil test is more detailed.

Points to Remember

Qualitative analysis finds if something is present or absent.
Dry method does not use water or dissolving.
Wet method dissolves the sample first.
Different tests show different elements.
Apparatus like test tubes and beakers are used for these tests.

2.2.2 Chemical Methods Of Quantitative Analysis

Quantitative analysis of organic compounds involves finding the percentage of each element and the concentration of known compounds. Quantitative analysis of simple inorganic compounds uses decomposition reaction (gravimetric analysis) and the progress of reaction between two solutions (titrametric or volumetric analysis). These methods measure mass and volume using equipment like weighing machines and burettes.

2.3 Mathematical Operation And Error Analysis

The accuracy of measurement is very important in analytical chemistry. There can be errors in analytical measurement. The numerical data from experiments are treated mathematically to reach quantitative conclusions. Therefore, an analytical chemist must know how to report quantitative analytical data, showing the accuracy of measurement, perform mathematical operations and properly express quantitative error in results.

2.3.2 Scientific Notation (Exponential Notation)

A chemist deals with very large numbers like 602,200,000,000,000,000,000,000 for molecules in 2 g of hydrogen gas or very small numbers like 0.00000000000000000000000166 g for the mass of a hydrogen atom. To avoid writing so many zeros, scientific notation is used. Any number can be written as N × 10ⁿ where n is an exponent with positive or negative values and N can be between 1 to 10. Thus, we can write 6.022 × 10²³ and 1.66 × 10^-24 g. The number 123.546 becomes 1.23546 × 10² in scientific notation. We move the decimal to the left by two places and the same number is the exponent (2) of 10. Similarly, 0.00015 can be written as 1.5 × 10^-4.

For adding 5.55 × 10⁴ and 6.95 × 10³, first make the exponent equal. Thus 5.55 × 10⁴ + 0.695 × 10⁴. Then add: (5.55 + 0.695) × 10⁴ = 6.245 × 10⁴

For subtraction of two numbers: 3.5 × 10^-2 - 5.8 × 10^-3 = (3.5 × 10^-2) - (0.58 × 10^-2) = (3.5 - 0.58) × 10^-2 = 2.92 × 10^-2

For multiplication: (5.6 × 10⁵) × (6.9 × 10⁸) = (5.6 × 6.9)(10⁵⁺⁸) = (5.6 × 6.9) × 10¹³ = 38.64 × 10¹³ = 3.864 × 10¹⁴

For multiplication: (9.8 × 10^-2) × (2.5 × 10^-6) = (9.8 × 2.5)(10^-2+(-6)) = (9.8 × 2.5) × (10^-2-6) = 24.50 × 10^-8 = 2.45 × 10^-7

Here the decimal must be moved four places to the right and (-4) is the exponent in scientific notation.

To perform addition, first write the numbers so they have the same exponent. Then add the coefficients.

The rule for multiplication of exponential numbers can be learned from the solved problems above.

Teacher's Note

Scientific notation helps us write very big or very small numbers easily. It is like writing 1 crore as 10⁷ instead of 10,000,000.

Exam Trick

Remember: When you move the decimal to the left, the exponent becomes positive. When you move it to the right, the exponent becomes negative. Count how many places you moved - that is your exponent power.

Points to Remember

Scientific notation writes numbers as N × 10ⁿ.
N must be between 1 and 10.
Moving decimal left gives positive exponent.
Moving decimal right gives negative exponent.
This method helps with very large and very small numbers.

2.3.2 Precision And Accuracy Of Measurement

The aim of measurement is to get the true value or accepted value of a quantity. Nearness of the measured value to the true value is called accuracy of measurement. Larger the accuracy, smaller the error. Accuracy depends on the sensitivity or least count (the smallest quantity that can be measured) of the measuring equipment.

Consider a burette reading of 10.2 mL. For all situations, the reading would be noted as 10.2 mL. This means there is uncertainty about the digit after the decimal point. This is because the least count of the burette is 0.1 mL. The reading 10.2 mL means the true value lies between 10.1 mL and 10.3 mL. This is written as 10.2 ± 0.1 mL. Here, the burette reading has an error of ± 0.1 mL.

Errors may be expressed as absolute or relative error.

Absolute error = Observed value - True value

Relative error is generally more useful than absolute error. Relative error is the ratio of absolute error to the true value. It is expressed as a percentage.

Relative error = \(\frac{\text{Absolute error}}{\text{True value}} \times 100\%\)

Error in measurement can happen for many reasons including inefficiency of the person doing measurement.

Multiple readings of the same quantity are noted to minimize error. If the readings match closely, they have high precision. High precision implies reproducibility of the readings. High precision is a prerequisite for high accuracy. Precision is expressed in terms of deviation. An absolute deviation is the difference between an observed value and the arithmetic mean for the set of several measurements.

Absolute deviation = Observed value - Mean

Arithmetic mean of all the absolute deviations is called the mean absolute deviation. The ratio of mean absolute deviation to its arithmetic mean is called relative deviation.

Relative deviation = \(\frac{\text{Mean absolute deviation}}{\text{Mean}} \times 100\%\)

In laboratory experiment, 10 g potassium chlorate sample on decomposition gives following data. The sample contains 3.8 g of oxygen and the actual mass of oxygen in the quantity of potassium chlorate is 3.92 g. The observed value is 3.8 g and accepted value is 3.92 g. Absolute error = Observed value - True value = 3.8 - 3.92 = -0.12 g. The negative sign indicates that experimental result is lower than the true value. The relative error = \(\frac{-0.12}{3.92} \times 100\%\) = -3.06%

Teacher's Note

Accuracy is like hitting the center of a target. Precision is like all your shots being very close together. In India, when chemists check water purity, they must be both accurate and precise.

Exam Trick

Remember: Accuracy = how close to true value. Precision = how close together measurements are. You can be precise but not accurate (like a broken weighing machine that always shows 500g even when weight is different).

Points to Remember

Accuracy is how close to the true value.
Precision is how close measurements are to each other.
Absolute error is the difference from true value.
Relative error shows error as a percentage.
High precision is needed for high accuracy.

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