Chapter 1: Some Basic Concepts of Chemistry

Q31Written answer

Hydrogen gas is prepared in the laboratory by reacting dilute HCl with

granulated zinc. Following reaction takes place.

Zn + 2HCl → ZnCl₂ + H₂

Calculate the volume of hydrogen gas liberated at STP when 32.65 g of zinc

reacts with HCl. 1 mol of a gas occupies 22.7 L volume at STP; atomic mass of

Zn = 65.3 u.

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Worked solution

1 mol of gas occupies = 22.7L Volume at STP atomic mass of Zn = 65.3u

From the above equation,

65.3g of Zn when reacts with HCl produces = 22.7L H2 at STP

Therefore, 32.65g of Zn when reacts with HCl will produce = 22.7 * 32.65/65.3 =11.35L of H2 at STP

Q32Written answer

The density of 3 molal solutions of NaOH is 1.110 g mL–1. Calculate the molarity

of the solution.

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Worked solution

3 molal solution of NaOH = 3 moles of NaOH dissolved in 1000g water

3 mole of NaOH = 3*40g = 120g

Density of solution = 1.110gmL-1

Volume = mass/density = 1120g/1.110gmL-1 =1.009L

Molarity of the solution = 3/1.009 = 2.97M

Q33Written answer

The volume of a solution changes with change in temperature, then, will the molality

of the solution be affected by temperature? Give a reason for your answer.

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Worked solution

Mass does not change as the temperature changes. Therefore, the molality of a solution does not change.

Molality = moles of solute/ weight of solvent (in g) *1000

Q34Written answer

If 4 g of NaOH dissolves in 36 g of H2O, calculate the mole fraction of each

component in the solution. Also, determine the molarity of the solution (specific

gravity of solution is 1g mL–1).

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Worked solution

Mole fraction of H2O = No; of moles of H2O/ Total no: of moles (H2O+NaOH)

No: of moles of H2O = 36/18=2moles

No: of moles of NaOH = 4/40=0.1mol

Total no: of moles = 2+0.1= 2.1

Mole fraction of H2O = 2/2.1 = 0.952

Mole fraction of NaOH = 0.1/2.1 = 0.048

Mass of solution = Mass of H2O + Mass of NaOH = 36+4=40G

Volume of the solution = 40/1 = 40mL

Molarity = 0.1/0.04 = 2.5M

Q35Written answer

The reactant which is entirely consumed in the reaction is known as limiting reagent.

In the reaction 2A + 4B → 3C + 4D, when 5 moles of A react with 6 moles of B,

then

(i) which is the limiting reagent?

(ii) calculate the amount of C formed?

Solution;

(i) B will be the limiting reagent as it gives a lesser amount of product.

(ii) Let B is completely consumed

4 mol B gives 3 mol C

6 mol B will give 3/4 *6 mol C =4.5 mol C

Match The Following Type

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Solution coming soon.

Q36Written answer

Match The Following Type

(i) 88 g of CO2

(ii) 6.022 ×1023 molecules of H2O

(iii) 5.6 litres of O2 at STP

(iv) 96 g of O2

(v) 1 mol of any gas

(a) 0.25 mol

(b) 2 mol

(c) 1 mol

(d) 6.022 × 10²³ molecules

(e) 3 mol

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Worked solution

A → b

B → c

C → a

D → e

E → d

Q37Written answer

Match the following

Physical quantity

Unit

(i) Molarity

(ii) Mole fraction

(iii) Mole

(iv) Molality

(v) Pressure

(vi) Luminous intensity

(vii) Density

(viii) Mass

(a) g mL–1

(b) mol

(c) Pascal

(d) Unitless

(e) mol L–1

(f) Candela

(g) mol kg–1

(h) Nm–1

(i) kg

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(i → e)

(ii → d)

(iii → b)

(iv → g)

(v → c)

(vi → f)

(vii → a)

(viii → i)

V. Assertion and Reason Type

In the following questions, a statement of Assertion (A) followed by a

statement of Reason (R) is given. Choose the correct option out of the

choices are given below each question.

Q38Written answer

Assertion (A): The empirical mass of ethene is half of its molecular mass.

Reason (R): The empirical formula represents the simplest whole-number

the ratio of various atoms present in a compound.

(i) Both A and R are true and R is the correct explanation of A.

(ii) A is true but R is false.

(iii) A is false but R is true.

(iv) Both A and R are false.

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Worked solution

Option (i) is correct.

Q39Written answer

Assertion (A): One atomic mass unit is defined as one-twelfth of the mass of

one carbon-12 atom.

Reason (R): Carbon-12 isotope is the most abundant isotope of carbon

and has been chosen as the standard.

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

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Worked solution

Option (ii) is correct. Carbon-12 is considered a standard for defining the atomic and molecular mass.

Q40Written answer

Assertion (A): Significant figures for 0.200 is 3 whereas for 200 it is 1.

Reason (R): Zero at the end or right of a number are significantly provided

they are not on the right side of the decimal point.

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not a correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

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Worked solution

Option (iii) is correct. Significant figures for 0.200 = 3 and for 200 =1

Zero at the end of a number without decimal point may or may not be significant depending on the accuracy of the measurement.

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