SOME BASIC CONCEPTS OF CHEMISTRY Flashcards
questions
NO and NO2 are two oxides of nitrogen. Which law of chemical combination is illustrated by these compounds? State the law. Who proposed this law?
Ans: **Law of Multiple proportion. **
It states that when two elements combine to form more than one compound, the different masses of one of the elements that combine with a fixed mass of the other, is in a simple whole number ratio. This law was proposed by John Dalton.
State Law of Definite (Constant) proportion. Who proposed this law?
: It states that a given compound always contains exactly the same proportion of elements by weight. This law was proposed by Joseph Proust
State Law of conservation of mass (matter). Who proposed this law?
: It states that matter can neither be created nor destroyed. Or, in a chemical reaction, the total mass of reactants is equal to the total mass of products. This law was proposed by
Antoine Lavoisier.
Define a.m.u
: 1/12th the mass of a C12 atom is called atomic mass unit (amu).
Define 1 mole.
1 mole is the amount of substance that contains as many particles as there are atoms in exactly 12 g C12 isotope.
OR
“A mole in chemistry is a unit used to count entities, like atoms or molecules. It’s based on Avogadro’s number, approximately 6.022 x 10^23, meaning one mole of a substance contains that many individual particles.”
6.022x10^23
6.022x10^23
Find the number of oxygen atoms present in 5 moles of glucose (C6H12O6)
: 1 mol glucose contains 6 mol Oxygen atoms.
So, 5 mol glucose contain 6 x 5 = 30 mol Oxygen atoms
= 30 x 6.022 x 1023 Oxygen atoms
Write the number of significant figures in the following:
(i) 0.0205 (ii) 3.00 x 10^5 (iii) 1005
(i) 3 (ii) 3 (iii) 4
Define mass percentage.
It is the percentage of each element present in 100g of a substance.
i.e. percentage composition (mass percent) of an element
=( Mass of that element in the compound/Molar mass of the compound) x 100
What are empirical and molecular formulae?
Empirical formula is the simplest formula which gives only the ratio of different
elements present in the compound. But molecular formula is the actual formula that gives
the exact number of different elements present in the sample
.A compound contains 4.07% hydrogen, 24.27% carbon and 71.65% chlorine. Its molecular mass is 98.96. What are the empirical and molecular formulae?
Element Percentage Atomic mass Percentage/
Atomic mass
C 24.27 12 24.27/12 = 2.02
H 4.07 1 4.07/1 = 4.07
Cl 71.65 35.5 71.65/35.5=2.02
Simple ratio Simplest whole
no. ratio
2.02/2.02 = 1 1
4.07/2.02 = 2 2
2.02/2.02 = 1 1
Empirical Formula = CH2Cl
Empirical Formula Mass (EFM) = 12+2+35.5 = 49.5
Molar mass (MM) = 98.96
n = MM/EFM = 98.96/49.5 = 2
Molecular formula = Empirical formula x n
= (CH2Cl) x 2 = C2H4Cl2
Find the number of moles and molecules present in 90 g water
No. of moles(n) = Given mass in gram (w) = 90 = 5 moles
Molar mass (M) 18
No. of molecules = no. of moles x 6.022x 1023 = 5 x 6.022x 1023 molecules
What is limiting reagent?
It is the reagent which limits a reaction. OR, it is the reagent which is completely
consumed in a chemical reaction
Differentiate between molarity and molality.
: Molarity is defined as the number of moles of solute dissolved per litre of solution.
OR,
Molarity (M) = Number of moles of solute (n)
Volume of solution in litre (V)
Molality is defined as the number of moles of the solute present per kilogram (kg) of the solvent.
OR, Molality (m) = Number of moles of solute
Mass of solvent in kg
Molality is temperature independent, while molarity changes with temperature.
Find the molarity of a solution prepared by dissolving 4g of NaOH dissolves in 250 mL
solution.
Molarity (M) = Number of moles of solute (n)
Volume of solution in litre (V)
No. of moles of NaOH = Given mass = 4 = 0.1 mol
Molar mass 40
Volume of solution in L = 250/1000 = 0.25 L
Molarity = 0.1/ 0.25 = 0.4M
