Chemistry chapter 1: stoichoimetry

Question 1

Define Avogadro’s number.

Answer 1

A: Avogadro’s number is
6.022×10^23, representing the number of atoms, molecules, ions or particles in one mole of a substance.

Question 2

What is a mole in chemistry?

Answer 2

A: A mole is the amount of substance that contains exactly
6.022×10^23
entities (atoms, molecules, ions, etc.), known as Avogadro’s number.

And it is the number of atomic, formula and molecular mass expressed in grams.

Question 2

Why is chemistry considered a quantitative science, and how does it impact daily life?

Answer 2

A: Chemistry is considered a quantitative science because it involves measurements, calculations, and precise quantities (moles, mass, volume, etc.) used to predict outcomes. It impacts daily life in areas like medicine, food, environment, and industrial processes.

Question 3

How is the mole concept related to Avogadro’s number?

Answer 3

A: One mole of any substance contains

6.022×10^23 particles, such as atoms, molecules, or ions, which is Avogadro’s number.

Question 4

How do you calculate the number of atoms, molecules, moles, ions, protons, neutrons, or electrons?

Answer 4

A: Use the formula:
Number of particles=moles×Avogadro’s number

For protons, neutrons, and electrons, use atomic number and mass number to find the counts.

Question 5

How do you calculate interacting moles and representative particles from a balanced chemical equation?

Answer 5

A: Use the mole ratio from the balanced equation to convert between moles of reactants and products.

Question 5

How do you calculate the masses and volumes of gases at STP and RTP?

Answer 5

A: Use the molar volume of gas: 22.4 L at STP and 24 L at RTP. The formula is:
Volume=Moles×Molar volume.

Question 6

How do you calculate the percentage by mass of elements in a compound?

Answer 6

A: Use the formula:

Percentage by mass= Mass of (element in 1 mole of compound/
Molar mass of compound)×100.

Question 6

How do you solve stoichiometric problems using mole ratios?

Answer 6

A: Use the mole ratio from the balanced equation as a conversion factor to find the moles of reactants or products.

Question 7

How do you calculate the percentage of water of crystallisation in hydrated salts?

Answer 7

A: Use the formula:

Percentage of water= (Mass of water/
Mass of hydrated salt)×100.

Question 8

How do you deduce the empirical and molecular formula of a compound?

Answer 8

A: Find the simplest whole number ratio of atoms (empirical formula).
Use the molar mass and empirical formula to find the molecular formula:

Molecular formula= n(Empirical formula)

Question 9

How do you determine the limiting reagent in a chemical reaction?

Answer 9

A: The limiting reagent is the reactant that is consumed first, limiting the amount of product formed. Compare the mole ratios of reactants to the balanced equation to determine this.

Question 10

How do you calculate the maximum amount of product and the amount of excess reagent left unreacted?

Answer 10

A: Use the limiting reagent to calculate the moles of product.
Subtract the moles of excess reagent used from the initial moles to find the unreacted amount.

Question 11

What is the difference between theoretical yield, actual yield, and percentage yield?

Answer 11

A:
- Theoretical yield is the maximum product calculated from the limiting reagent.
- Actual yield is the product obtained in practice.

• Percentage yield= (Actual yield/Theoretical yield)×100.

Question 12

How many molecules are present in 2 moles of water (H₂O)?

Answer 12

A:
Use Avogadro’s number:
1 mole=6.022×10^23 molecules.
Multiply by the number of moles:
2 moles of H₂O×6.022×10^23 =1.204×10^24 molecules.

Question 13

Calculate the percentage of carbon in methane (CH₄).

Answer 13

Find the molar mass of CH₄:
12+(1×4)=16 g/mol

Use the formula for percentage composition:
Percentage of carbon
​
(12/16)×100=75%.

Question 14

In the reaction
2H2+O2→2H2O,
, if you have 3 moles of H₂ and 2 moles of O₂, what is the limiting reagent?

Answer 14

A:

• According to the balanced equation, 2 moles of H₂ react with 1 mole of O₂.
• For 3 moles of H₂, you would need
  3/2=1.5 moles of O₂
• Since you have 2 moles of O₂ (more than required), H₂ is the limiting reagent.

Question 15

Define theoretical yield and actual yield in chemical reactions.

Answer 15

Theoretical yield is the maximum amount of product that can be produced from the limiting reagent, based on the stoichiometric calculations.
Actual yield is the amount of product actually obtained from a chemical reaction.

Question 16

How do mole ratios help in solving stoichiometry problems?

Answer 16

A: Mole ratios, derived from the coefficients of a balanced chemical equation, act as a conversion factor to relate the moles of reactants and products.

Question 17

What is the percentage of oxygen in water (H₂O)?
a) 11.11%
b) 88.89%
c) 33.33%
d) 50%

Answer 17

A: b) 88.89%

Question 17

What volume will 2 moles of oxygen gas occupy at STP?
a) 11.2 L
b) 22.4 L
c) 44.8 L
d) 24 L

Answer 17

A: c) 44.8 L
At STP, 1 mole of gas = 22.4 L, so 2 moles = 44.8 L

Question 18

How many moles are in
18.06 x 10^23 atoms of carbon?
a) 0.3 moles
b) 1.5 moles
c) 3 moles
d) 6 moles

Answer 18

A: c) 3 moles

Question 19

What is an empirical formula, and how is it different from a molecular formula?

Answer 19

• Empirical formula is the simplest whole-number ratio of atoms in a compound.
• Molecular formula shows the actual number of atoms of each element in a molecule.

Question 20

How is Avogadro’s number applied in chemical calculations?

Answer 20

: Avogadro’s number is used to convert between moles and the number of atoms, molecules, or ions in a substance, aiding in quantitative chemical analysis.

Question 20

in reaction
N2 + 3H2 –> 2NH3
, if 1 mole of nitrogen reacts with 3 moles of hydrogen, what is the limiting reagent?
a) Nitrogen
b) Hydrogen
c) Ammonia
d) None

Answer 20

A: d) None
The reactants are present in the exact stoichiometric ratio, so neither is limiting.

Question 21

in reaction:
2H2 + O2 —> 2H2O, how many moles of water are produced when 4 moles of hydrogen react?

Answer 21

According to the balanced equation, 2 moles of H₂ produce 2 moles of H₂O.
Therefore, 4 moles of H₂ will produce 4 moles of H₂O.

Question 22

Calculate the volume of 10 g of oxygen gas (O₂) at STP.

Answer 22

• Find the molar mass of O₂:
  32 g/mol.
• Calculate the moles: 10/32
  =0.3125 moles.
• Use the molar volume at STP:
  Volume=0.3125×22.4=7 L.

Question 23

chemistry?

Answer 23

branch of science that deals with the study of properties, structure, composition and the changes that occur in matter along with the laws that govern these changes.

Question 24

compounds

Answer 24

a substance formed by the chemical bonding of elements

Question 25

elements

Answer 25

a substance made up of one type of atoms having the same proton number

Question 26

ion

Answer 26

charged atom.
- cation: positively charged
- anion: negatively charged

Question 27

Stoichiometry

Answer 27

Branch of chemistry that deals with the QUANTITATIVE measures of the relationship between the reactants and the products in a balanced chemical equation.