Concept de Mole et Stœchiométrie

Question 1

Define (1) relative atomic mass of an element, (2) relative isotopic mass of a particular isotope, (3) relative molecular mass of a molecule and (4) relative formula mass of an ionic compound

Answer 1

1. The ratio of the average mass of one atom of that element to 1/12 of the mass of an atom of carbon-12 (Symbol: Ar)
2. The ratio of the mass of one atom of that isotope to 1/12 the mass of an atom of carbon-12
3. The ratio of the average mass of one molecule to 1/12 the mass of an atom of carbon-12 (Symbol: Mr)
4. The ratio of the average mass of one formula unit of that compound to 1/12 the mass of an atom of carbon-12 (Symbol: Mr)

Question 2

Define mole

Answer 2

The amount of substance containing exactly 6.02 x 10^23 (Avogadro’s constant) elementary entities (or particles)

Question 3

List the formulae in mole concept

Answer 3

n is amt/no. of moles

1. n=N/L , N=no. of particles & L=Avogadro constant (no. of particles/mol)
2. n=m/M ,M=molar mass (g mol-1)
3. (Gas oni) n=V/Vm , Vm=molar volume (dm3 mol-1)
4. conc in g dm-3=m/V
5. [ ]=n/V
6. conc in g dm-3=[ ] x M , M=molar mass (g mol-1)

Question 4

What is molar mass of a substance and its unit?

Answer 4

Mass of 1mol of that substance
Unit: g mol-1

Question 5

What is standard sf for mole calculations?

Answer 5

3sf
(Unless stated otherwise)

Question 6

What is molar volume?

Answer 6

Volume occupied by 1mol of a gas

Question 7

What is molar volume at stp and rtp

Answer 7

stp: 22.7dm3 mol-1

rtp: 24.0dm3 mol-1

Question 8

Avogadro’s Law implies that… (gases)

Answer 8

Mole ratio = Volume ratio

Question 9

Percentage yield=?

Answer 9

actual yield/theoretical yield X 100%

Question 10

Limiting reagent always determines what?

Answer 10

Actual amt of product formed

Question 11

What remains constant during dilution?

Answer 11

Amt of solute in solution

Question 12

What is the formula for dilution concept?

Answer 12

C1V1=C2V2 ,

C is conc, V is vol, 1 is original, 2 is diluted

Question 13

Percentage composition/purity=?

Answer 13

mass of pure substance/mass of sample x100%

Question 14

What is empirical formula and molecular formula?

Answer 14

Empirical:
simplest formula showing ratio of no of atoms of each element present in a compound

Molecular:
Exact formula showing actual no of atoms of each element present in a compound

Question 15

How is molecular formula obtained?

Answer 15

Mr=n x sum of Ar in empirical formula (n is integer)

Question 16

What is the general combustion equation for hydrocarbons?

Answer 16

CxHy + (x+y/4)O2 —> xCO2 + (y/2)H2O

Question 17

What do alkalis do to acidic gases?

Answer 17

Alkalis absorb acidic gases

Question 18

What is ppm?

Answer 18

Parts per million, of a particular quantity (eg. 1kg, 1dm3)

Question 19

What is the formula for scaling up/down concept?

Answer 19

Let A be a substance in 250cm3

if amt of A in 250cm3=y mol ,

Down (decrease amt):
amt of A in 25cm3=(25/250)y mol

Up (increase amt):
amt of A in 1000cm3=(1000/250)y mol

Question 20

What remains constant during scaling up/down

Answer 20

Concentration of solute