Quantative Chemistry

Question 1

What is relative formula mass?

Answer 1

The sum of the relative atomic masses of all the atoms in the molecular formula

Question 2

What is the formula for percentage mass of an element in a compound?

Answer 2

Percentage mass = ((Atomic mass x Number of atoms)/Relative formula mass) x 100

Question 3

What is the percentage mass of sodium in sodium carbonate Na2CO3?

Answer 3

43%

Question 4

How much is a mole?

Answer 4

6.02 x 10^23

Question 5

How does the mole relate to particles in a substance?

Answer 5

One mole of any substance is the amount of that substance that contains 6.02 x 10^23 particles. This could be atoms, molecules, ions or electrons

Question 6

Why is a mole that number?

Answer 6

One mole of atoms of any substance will have a mass in grams equal to the relative formula mass

Question 7

What is the formula for the number of moles in a given mass?

Answer 7

Number of moles = mass(g) / Relative formula mass

Question 8

What is the conservation of mass theory?

Answer 8

Atoms/mass cannot be created or destroyed so there are the same number and type if atoms on each side of a reaction equation

Question 9

What does it mean if the mass of the products is larger than the mass of the reactants?

Answer 9

One of the reactants is a gas found in the air but all the products are solids, liquids or aqueous because the gas is part of the reaction

Question 10

What does it mean if the mass of the products is smaller than the mass of the reactants?

Answer 10

One of the products is a gas and all the reactants are solids, liquids or aqueous and the gas escaped during the reaction.

Question 11

What do the big numbers in front of the chemical formulas of the reactants and products tell you?

Answer 11

How many moles of each substance take part in the reaction

Question 12

How do you work out a balanced symbol equation using the masses of the products and reactants?

Answer 12

-Divide each substance’s mass by its relative formula mass to find the number of moles
-Divide all of these by the smallest number of moles in the reaction
-(If there are decimals) Multiply everything out so all of the numbers are whole
-Put these numbers in front of the chemical formulas

Question 13

What is the limiting reactant in a reaction?

Answer 13

The reactant that ends the reaction by being used all the way up. The amount of product formed is normally directly proportional to it

Question 14

How can you use the limiting reactant and balanced equation to calculate the amount of product formed?

Answer 14

-write out the balanced equation
-work out the relative formula mass of the desired reactant and product
-find out the number of moles of the substance you know the mass of
-use the equation to work out how many moles there will be of the other substance
-use this to calculate the mass

Question 15

What is the theorectical yield of a reaction?

Answer 15

The mass of product that you calculate (theoretical because you will never get 100% of the product)

Question 16

At room temperature (20 degrees celsius) and room pressure (1 atm) what volume will one mole of any gas occupy?

Answer 16

24dm^3

Question 17

What is the formula for finding the volume of a known mass of any gas at room temperature (r.t.p.)?

Answer 17

Volume of gas = (Mass of gas / Relative formula mass of gas) x 24

Question 18

What is the formula for finding the concentration of a substance in a solution using mass?

Answer 18

Concentration = mass of the solute / volume of the solvent

Question 19

What is the formula to convert the units of a concentration from mol/dm^3 to g/dm^3?

Answer 19

Mass = moles x Mr

Question 19

What is the formula for finding the concentration of a substance in a solution using moles?

Answer 19

Concentration = number of moles of solute / volume of solvent

Question 20

What are the negatives of reactions with low atom economies?

Answer 20

-use up resources
-make waste materials that need disposing
-unsustainable
-not profitable

Question 21

What does the atom economy of a reaction tell you?

Answer 21

How much of the mass of the reactants is wasted when manufacturing a chemical and how much ends up as useful products. 100% atom economy means all the atoms have been turned into useful products

Question 22

What is the formula for atom economy?

Answer 22

Atom economy = (relative formula mass of desired product / relative formula mass of all reactants) x 100

Question 23

What is the yield of a reaction?

Answer 23

The amount of product that is made

Question 23

What are the factors to consider when choosing which reaction to use to make your product?

Answer 23

-atom economy
-yield
-rate of reaction
-position of equilibrium (in reversible reactions)

Question 24

What is the best way around the negatives of reactions with low atom economies?

Answer 24

Finding a use for the waste products or using an alternative reaction that still makes the product you want but also makes useful by-products instead of waste products

Question 25

What is the formula for the percentage yield of a reaction?

Answer 25

Percentage yield = (mass of product actually made / maximum theoretical mass of the product) x 100

Question 26

What are the benefits of a high percentage yield?

Answer 26

-reduced waste
-reduced cost

Question 27

What are the three most common ways for percentage yield to be decreased in a reaction?

Answer 27

-not all reactants react to make the product (reversible reactions)
-side reactions (reactants may react with gases in the air or impurities in the mixture)
-product lost during separation from the reaction mixture (e.g. during filtration)

Question 28

Why can you never get 100% yield in real life?

Answer 28

Because some product or reactant will always get lost along the way somehow