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Chemistry Answer the following in one or two sentences.

Introduction to Analytical Chemistry · Maharashtra Board STD 11 (MSBSHSE - Semi English Medium). 17 questions.

Questions

Answer the following in one or two sentences.

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17 questions · self-marked practice — reveal the answer and mark yourself.

Question 12 Marks
Why there is a need of rounding off figures during calculation ?
Answer
When performing calculations with measured quantities, the rule is that the accuracy of the final result is limited to the accuracy of the least accurate measurement.
In other words, the final result cannot be more accurate than the least accurate number involved in the calculation.
Sometimes, the final result of a calculation often contains figures that are not significant.
When this occurs, the final result is rounded off.
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Question 22 Marks
State TRUE or FALSE. Correct the statement if false.
i. A majority of reactions in the laboratory are carried out in in gaseous forms.
ii. Molarity is the most widely used to express the concentration of solution.
iii. Molality of a solution changes with temperature.
Answer
i. False:- A majority of reactions in the laboratory are carried out in solution forms.
ii. True
iii. False:- Molality of a solution does not change with temperature.
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Question 32 Marks
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. Calculate absolute error and relative error.
Answer
The observed value is 3.8 g and accepted (true) value is 3.92 g.
i. Absolute error = Observed value – True value
= 3.8 – 3.92
= -0.12 g
$\begin{aligned} & \text { ii. } \quad \text { Relative error }=\frac{\text { Absolute error }}{\text { True value }} \times 100 \% \\ & =\frac{-0.12}{3.92} \times 100 \% \\ & \end{aligned}$
$=-3.06 \%$
[Note: The negative sign indicates that experimental result is lower than the true value.]
Ans: i. Absolute error = -0.12 g
ii. Relative error = -3.06%
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Question 42 Marks
Subtract $6.90 \times 10^{-5}$ from $5.11 \times 10^{-4}$ and express the result in scientific
Answer
To perform subtraction operation, first the numbers are written in such a way that they have the same exponent. Then subtraction of coefficients can be done.
$(5.11 \times 10^{-4}) – (6.90 \times 10^{-5})$
$= (5.11 \times 10^{-4}) – (0.690 \times 10^{-4})$
$= (5.11 – 0.690) \times 10^{-4}$
$= 4.42 \times 10^{-4}$
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Question 52 Marks
Subtract $5.8 \times 10^{-3}$ from $3.5 \times 10^{-2}$ and express result in scientific notation.
Answer
To perform subtraction operation, first the numbers are written in such a way that they have the same exponent. Then subtraction of coefficients can be done.
$(3.5 \times 10^{-2})- (5.8 \times 10^{-3})$
$= (3.5 \times 10^{-2}) – (0.58 \times 10^{-2})$
$= (3.5 – 0.58) \times 10^{-2}$
$= 2.92 \times 10^{-2}$
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Question 62 Marks
Add $1.77 \times 10^2$ and $2.23 \times 10^3$ and express the result in scientific notation.
Answer
To perform addition operation, first the numbers are written in such a way that they have the same exponent. The coefficients are then added.
$(1.77 \times 10^2) + (2.23 \times 10^3)$
$= (0.177 \times 10^3) + (2.23 \times 10^3)$
$= (0.177 + 2.23) \times 10^3$
$= 2.407 \times 10^3$
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Question 72 Marks
Add $5.55 \times 10^4 $and $6.95 \times 10^3$ and express the result in scientific notation.
Answer
To perform addition operation, first the numbers are written in such a way that they have the same exponent. The coefficients are then added.
$(5.55 \times 10^4) + (6.95 \times 10^3)$
$= (5.55 \times 10^4) + (0.695 \times 10^4)$
$= (5.55 +0.695) \times 10^4$
$= 6.245 \times 10^4$
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Question 82 Marks
State the rules used to round off a number to the required number of significant figures.
Answer
The following rules are used to round off a number to the required number of significant figures:
  • If the digit following the last digit to be kept is less than five, the last digit is left unchanged, e.g. 46.32 rounded off to two significant figures is 46.
  • If the digit following the last digit to be kept is five or more, the last digit to be kept is increased by one. e.g. 52.87 rounded to three significant figures is 52.9.
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Question 92 Marks
How many significant figures are present in each of the following?
$i. \ 45.0  \ \ ii. \ 0.001 \ \  iii. \ 2.10 \times 10^{-8} \ \  iv. \ 340000 \ \ v. \ 0.0100 \ \  vi. \ 7890320 \ \ vii. \ 100.00 \ \ viii. \ 100$
Answer
$i. \ 3  \ \  ii. \ 1  \ \ iii. \ 3 \ \  iv. \ 2 \ \  v. \ 3 \ \ vi. \ 6 \ \ vii. \ 5 \ \ viii. \ 1$
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Question 102 Marks
How many significant figures are present in the following measurements?
$i. \  4.065 \ m \ \ ii. \  0.32 g \ \ iii. \ 57.98 \ cm^3 \ \ iv. \ 0.02 s \ \ v. \ 4.0 \times 10^{-4} \ km \ \ vi. \ 604.0820 \ kg \ \ vii. \ 307.100 \times 10^{-5} \ cm$
Answer
$i. \ 4 \ \ ii. 2 \ \ iii. \ 4 \ \ iv. \ 1 \ \ v. \ 2 vi. \ 7 \ \ vii. \ 6$
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Question 112 Marks
Explain the term: Relative error
Answer
Relative error is the ratio of an absolute error to the true value.
Relative error is generally a more useful quantity than absolute error.
Relative error is expressed as a percentage and can be calculated as follows:
Relative error $=\frac{\text { Absolute error }}{\text { True value }} \times 100 \%$
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Question 122 Marks
Why are scientific notations (exponential notations) used?
Answer
A chemist has to deal with numbers as large as 602,200,000,000,000,000,000,000 for the molecules of 2 g of hydrogen gas or as small as 0.00000000000000000000000166 g. that is, mass of a H atom. To avoid the writing of so many zeros in mathematical operations, scientific notations i.e. exponential notations are used.
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Question 132 Marks
What are the two stages involved in the chemical analysis of a sample?
Answer
The chemical analysis of a sample is carried out mainly in two stages: by the dry method and by the wet method. In dry method, the sample under test is not dissolved and in wet method, the sample under test is first dissolved and then analysed to determine its composition.
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Question 142 Marks
What does classical qualitative analysis method include?
Answer
Classical qualitative analysis method includes separation and identification of compounds.
  • Separations may be done by methods such as precipitation, extraction and distillation.
  • Identification may be based on differences in colour, odour, melting point, boiling point, and reactivity.
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Question 152 Marks
What is semi-microanalysis?
Answer
  • When the amount of a solid or liquid sample taken for analysis is a few grams, the analysis is called semi-microanalysis.
  • It is of two types: qualitative and quantitative analysis.
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Question 162 Marks
What is the difference between qualitative analysis and quantitative analysis?
Answer
  1. Qualitative analysis deals with the detection of the presence or absence of elements in compounds and of chemical compounds in mixtures.
  2. Quantitative analysis deals with the determination of the relative proportions of elements in compounds and of chemical compounds in mixtures.
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Question 172 Marks
Explain the statement: Analytical chemistry provides physical or chemical information about a sample.
Answer
  • Analytical chemistry facilitates the investigation of the chemical composition of substances.
  • It uses the instruments and methods to separate, identify and quantify a sample under study.
Thus, analytical chemistry provides chemical or physical information about a sample.
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