Chapter 3- Stoichiometry

Question 1

What is a chemical reaction?

Answer 1

The formation of new substances from one or more starting materials

Question 2

What is stoichiometry concerned with?

Answer 2

The relative amounts of products and reactants

Question 3

Why is it important to balance chemical equations?

Answer 3

It ensures the same number of atoms of each element appear in the products as in the reactants, thus conforming to the laws of conservation of mass

Question 4

What is the mole?

Answer 4

It’s the base SI for amount. It’s the account of any substance with the same number of atoms/molecules as 12g of 12C

Question 5

Give Avogadro’s constant

Answer 5

6.022 × 10^23 mol^-1

The amount of specified entities (atom/molecules etc) per mole

Question 6

Define the term ‘molar mass’

Answer 6

The mass of one mole of a substance, in g mol^-1

Question 7

Moles=

Answer 7

Mass (g)/ Molar mass (g mol^-1)

Question 8

Why is the percentage yield used?

Answer 8

To estimate the efficiency of a chemical reaction

Question 9

Percentage yield=

Answer 9

(actual yield/theoretical yield) x 100

Question 10

Define the term ‘limiting reagent’

Answer 10

The substance that is totally consumed when the chemical reaction is complete. The amount of product formed is limited by this reagent, since the reaction cannot continue without it.

Question 11

Define the term ‘excess reagent(s)’

Answer 11

If one or more other reagents are present in excess of the quantities required to react with the limiting reagent.

Question 12

How do you work out which reagent is the limiting reagent?

Answer 12

1. Find the moles of each reactant present.
2. Calculate the moles of a product formed from each mole of reactant.
3. Identify the reactant giving the smaller number of moles of product. This reactant is the limiting reagent

Question 13

How do you work out the theoretical yield?

Answer 13

The theoretical yield is the amount of the product in g formed from the limiting reagent. From the moles of limiting reagent available, calculate the grams of product that is theoretically possible

Question 14

State the difference between the empirical formula and the molecular formula

Answer 14

Empirical formulas show the simplest whole number ratio of these atoms. Molecular formulas show the actual ratio of atoms in the compound.
Example
Glucose’s molecular formula is C6H12O6. Its empirical formula is CH2O

Question 15

Give the formula to work out mass concentration

Answer 15

mass (g)/volume (dm^3)

Unit= g dm^-3

Question 16

Give the formula to work out molar concentration

Answer 16

moles/volume (dm^3)

Unit= mol dm^-3

Question 17

How do you convert cm^3 to dm^3?

Answer 17

divide by 1,000

Question 18

Define the term ‘reactant’

Answer 18

A substance that takes part in and undergoes change during a reaction.

Question 19

Define the term ‘product’

Answer 19

A product is a substance that is formed as the result of a chemical reaction

Question 20

What are stoichiometric coefficients?

Answer 20

Stoichiometric coefficients are the numbers that appear before the symbol for each compound in the equation for a chemical reaction

Question 21

Write a balanced equation for the complete combustion of maltose (C12H22O11)

Answer 21

C12H22O11 + 12O2 → 11H2O + 12CO2