Some Basic Concepts of Chemistry

### Convert the following into basic units: (i) 28.7 pm (ii) 15.15 pm (iii) 25365 mg

(i) 28.7 pm $1 pm = 10^{ -12 } \; m$ $28.7 pm = 28.7 \times 10^{ -12 } \; m$ $= 2.87 \times 10^{ -11 } \; m$   (ii) 15.15 pm $1 pm = 10^{ -12 } \; m$ $15.15 pm = 15.15 \times 10^{ -12 } \; m$...

### A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Find: (i) Empirical formula (ii) Molar mass of the gas, and (iii) Molecular formula

(i) Empirical formula 1 mole of $CO_{ 2 }$ contains 12 g of carbon Therefore, 3.38 g of $CO_{ 2 }$ will contain carbon = $\frac{ 12 \; g }{ 44 \; g } \; \times 3.38 \; g$ = 0.9217 g   18 g of...

### Calculate the concentration of nitric acid in moles per litre in a sample which has a density, 1.41 g mL–1 and the mass per cent of nitric acid in it being 69%

Mass percent of HNO3 in sample is 69 % Thus, 100 g of HNO3 contains 69 g of HNO3 by mass. Molar mass of HNO3 = { 1 + 14 + 3(16)} g.mol^{-1}g.mol−1 = 1 + 14 + 48 = 63g mol^{-1}=63gmol−1   Now,...

### Define the law of multiple proportions. Explain it with two examples. How does this law point to the existence of atoms?

When two elements combine to form two or more chemical compounds, then the mass of one of the compounds in a fixed mass of the other holds a simple measure of each other is the law of equality....

### Calcium carbonate reacts with aqueous HCl to give CaCl2 and CO2 according to the reaction given below:

CaCO3 (s) + 2HCl (aq) → CaCl2(aq) + CO2(g) + H2O(l) What mass of CaCl2 will be formed when 250 mL of 0.76 M HCl reacts with 1000 g of CaCO3? Name the limiting reagent. Calculate the number of moles...

### A vessel contains 1.6 g of dioxygen at STP (273.15K, 1 atm pressure). The gas is now transferred to another vessel at a constant temperature, where the pressure becomes half of the original pressure. Calculate: (i) the volume of the new vessel. (ii) a number of molecules of dioxygen.

(i) Calculation: Moles of oxygen = 1.6/32 Moles of oxygen = 0.05mol 1 mol of oxygen= 22.4L (at STP) Volume of Oxygen (V1) = 22.4 × 0.05 Volume of Oxygen (V1) = 1.12L V2 =? P1 = 1atm P2 = ½ P2 =...

### 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.

Correct Answer: (ii) Both A and R are true but R is not the correct explanation of A Explanation: The carbon 12 isotope defines the mass of atoms and molecules.

### 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.

Correct Answer: (i) Both A and R are true and R is the correct explanation of A Explanation: The empirical formula represents the simplest whole-number the ratio of various atoms present in a...

### Match the following

Physical quantity Unit (i) Molarity (a) g mL–1 (ii) Mole fraction (b) mol (iii) Mole (c) Pascal (iv) Molality (d) Unitless (v) Pressure (e) mol L–1 (vi) Luminous intensity (e) mol L–1 (vii) Density...

### Match the following

(i) 88 g of CO2 (a) 0.25 mol (ii) 6.022 ×1023 molecules of H2O (b) 2 mol (iii) 5.6 litres of O2 at STP (c) 1 mol (iv) 96 g of O2 (d) 6.022 × 1023 molecules (v) 1 mol of any gas (e) 3 mol  ...

### 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).

Calculation: Mole fraction of H2O = Number of moles of H2O / Total number of moles (H2O +NaOH) Number of moles of H2O = 36/18 Number of moles of H2O =2 moles Number of moles of NaOH = 4/40 Number of...

### 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.

The Mass do not change when the temperature changes and so the molality of a solution do not change as well. Molality of a substance is defined as the number of mass of solute per mass of the...

### 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.  ...

### If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in whole-number ratio. (a) Is this statement true? (b) If yes, according to which law? (c) Give one example related to this law.

(a) If two elements can combine to form more than one compound, the masses of one element that combine with a fixed mass of the other element, are in whole-number ratio and this statement is true....

### Calculate the mass percent of calcium, phosphorus and oxygen in calcium phosphate Ca3(PO4)

Calculation: Molecular mass of Ca3(PO4) = (3 X 40) + (2 X 31) + (8 X 16) Molecular mass of Ca3(PO4) = 310 Mass percentage of Ca = $\frac{3\times 40}{310\times 100}$ Mass percentage of Ca = 38.71%...

### What is the difference between molality and molarity?

Molarity Molality Number of moles per volume of the solution in litres Number of mass of solute per mass of the solvent in liters Unit - M Unit - m

### What is the symbol for the SI unit of a mole? How is the mole defined?

Mole is the amount of substance containing more entities because there are atoms in 12 g of carbon. SI unit symbol – mol

### How many significant figures should be present in the answer of the following calculations? 2.5×1.25×3.5/ 2.01

Number Significant figures 2.5 2 1.25 3 3.5 2 2.01 3 In the given calculation, involving both the multiplication and division, the significant figures present is 2. Hence, the result cannot have...

### What will be the mass of one atom of C-12 in grams?

1 mole of carbon atom = 12g Therefore, Mass of one atom of C-12 in grams= 1.99 × 1023 grams.

### One of the statements of Dalton’s atomic theory is given below: “Compounds are formed when atoms of different elements combine in a fixed ratio” Which of the following laws is not related to this statement? (i) Law of conservation of mass (ii) Law of definite proportions (iii) Law of multiple proportions (iv) Avogadro’s law

Correct Answers: (i) Law of conservation of mass; (iv) Avogadro's law Explanation: According to the Dalton's atomic theory, The Chemical compounds are formed when atoms of various elements join in a...

### Which of the following solutions have the same concentration?

(i) 20 g of NaOH in 200 mL of solution (ii) 0.5 mol of KCl in 200 mL of solution (iii) 40 g of NaOH in 100 mL of solution (iv) 20 g of KOH in 200 mL of solution   Answer: Correct Answers: (i)...

### Which of the following pairs have the same number of atoms?

(i) 16 g of O2(g) and 4 g of H2(g) (ii) 16 g of O2 and 44 g of CO2 (iii) 28 g of N2 and 32 g of O2 (iv) 12 g of C(s) and 23 g of Na(s)   Answer: Correct Answers: (iii) 28 g of N2 and 32 g of...

### Sulphuric acid reacts with sodium hydroxide as follows:

H2SO4 + 2NaOH → Na2SO4+ 2H2O When 1L of 0.1M sulphuric acid solution is allowed to react with 1L of 0.1M sodium hydroxide solution, the amount of sodium sulphate formed and its molarity in the...

### One mole of oxygen gas at STP is equal to _______.

(i) 6.022 × 1023 molecules of oxygen (ii) 6.022 × 1023 atoms of oxygen (iii) 16 g of oxygen (iv) 32 g of oxygen   Answer: Correct Answers: (i) 6.022 × 1023 molecules of oxygen; (iv) 32 g of...

### Which of the following statements indicates that the law of multiple proportions is being followed.

(i) Sample of carbon dioxide taken from any source will always have carbon and oxygen in the ratio 1:2. (ii) Carbon forms two oxides namely CO2 and CO, where masses of oxygen which combine with a...

### Which of the following reactions is not correct according to the law of conservation of mass.

(i) 2Mg(s) + O2(g) →2MgO(s) (ii) C3H8(g) + O2(g) → CO2(g) + H2O(g) (iii) P4(s) + 5O2(g) → P4O10(s) (iv) CH4(g) + 2O2(g) → CO2(g) + 2H2O (g)   Answer: Correct Answer: (ii) C3H8(g) + O2(g) →...

### Which of the following statements is correct about the reaction given below: 4Fe(s) + 3O2(g) → 2Fe2O3(g) (i) The total mass of iron and oxygen in reactants = total mass of iron and oxygen in product therefore it follows the law of conservation of mass. (ii) The total mass of reactants = total mass of product; therefore, the law of multiple proportions is followed. (iii) Amount of Fe2O3 can be increased by taking any one of the reactants (iron or oxygen) in excess. (iv) Amount of Fe2O3 produced will decrease if the amount of any one of the reactants (iron or oxygen) is taken in excess.

Correct Answer: (i) The total mass of iron and oxygen in reactants = total mass of iron and oxygen in product therefore it follows the law of conservation of mass. Explanation: From the reaction,...

### Which of the following statements about a compound is incorrect? (i) A molecule of a compound has atoms of different elements. (ii) A compound cannot be separated into its constituent elements by physical methods of separation. (iii) A compound retains the physical properties of its constituent elements. (iv) The ratio of atoms of different elements in a compound is fixed.

Correct Answer: (iii) A compound retains the physical properties of its constituent elements Explanation: Molecule of a compound is made up of atoms of various elements which cannot be separated...

### If the density of a solution is 3.12 g mL-1, the mass of 1.5 mL solution in significant figures is _______. (i) 4.7g (ii) 4680 × 10 -3g (iii) 4.680g (iv) 46.80g

Correct Answer: (i) 4.7g Explanation: Given, Density of solution = 3.12 g mL-1 Volume of solution = 1.5 mL Formula for Mass = Volume × Density Hence, Mass = 4.7 g

### The empirical formula and molecular mass of a compound are CH2O and 180 g respectively. What will be the molecular formula of the compound? (i) C9H18O9 (ii) CH2O (iii) C6H12O6 (iv) C2H4O2

Correct Answer: (iii) C6H12O6 Explanation: Given, Molar mass of Carbon=12 Molar mass of Hydrogen=1 Molar mass of Oxygen=16 So, The molecular weight of compound is 6 and so the molecular formula of...