Some Basic Concepts

Question 1

Law of Conservation of mass

Answer 1

Matter can neither be created nor destroyed

Question 2

Law of definite proportions

Answer 2

A given compound always contains exactly the same proportion of elements by weight

Question 3

Law of Multiple Proportions

Answer 3

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 the ratio of small whole numbers

Question 4

Gay Lussac’s Law

Answer 4

When gases combine or are produced in a chemical reaction they do so in a simple ratio by volume, provided all gases are at same temperature and pressure

Question 5

Avogadro’s Law

Answer 5

Equal volumes of all gases at the same temperature and pressure should contain equal number of molecules

Question 6

Amount present in one mole

Answer 6

Avogadro Number - 6.022*(10^23)

Question 7

Mass by mass percentage

Answer 7

(Mass of element / Mass of compound)*(100)

Question 7

Finding molecular formula

Answer 7

Molar mass / Empirical formula = n
Putting n moles in empirical formula

Question 7

Steps for calculating empirical formulas

Answer 7

1. Take percentage as in grams
2. Find moles of each element
3. Simplify in simplest whole number ratio

Question 8

Limiting reagent

Answer 8

Reactant, which gets consumed first, limits the amount of product formed

Question 9

Mole Fraction

Answer 9

Ratio of number of moles of a particular component to the total number of moles in the solution
(Na / (Na + Nb))

Question 10

Molarity

Answer 10

M =Number of moles of solute / (Volume of solution in Litres)

Question 11

Formula for molarity (when two solutions)

Answer 11

M1V1 = M2V2

Question 12

Molality

Answer 12

m = no. of moles of solute / mass of solvent in kg

Question 13

If the density of methanol is 0.793 kg L–1, what is its volume needed for making 2.5
L of its 0.25 M solution?

Answer 13

Using, M1V1 = M2V2:
0.793/0.032 = 24.78M
24.78V1 = 0.25 * 2.5L
V1 = 25.22mL

Question 14

Calculate the molarity of a solution of ethanol in water, in which the mole fraction of
ethanol is 0.040 (assume the density of water to be one).

Answer 14

2.314M