Quantitative chemistry

Question 1

Percentage mass of an element in a compound=

Answer 1

Ar x number of that element/ Mr of that compound
x100

Question 2

What is Mr

Answer 2

relative formula mass
-which is the relative atomic masses of all the atoms in the molecular formula added tg

Question 3

What is the Avogadro constant

Answer 3

6.02 x 10^23

Question 4

What is one mole of any substance

Answer 4

the amount of that substance that contains an avogadro number of particles
-the particles cld be atoms ,molecules, ions or electrons

Question 5

One mole of atoms or molecules of any substance will have..

Answer 5

a mass in grams equal to the relative formula mass (Ar or Mr) for that substance

Question 6

EG. Carbon has an Ar of 12 so..

Answer 6

one mole of carbon weighs exactly 12g

Question 7

EG. CO2 has a Mr of 44 (12+ (2x16) so…

Answer 7

one mole of CO2 weighs exactly 44g

Question 8

Formula to find the number of moles in a given mass

Answer 8

number of moles =
mass in g(of an element/compound)
DIVIDED BY
Mr (of the element/compound)

Question 9

What doesn’t happen during a chemical reaction with atoms

Answer 9

-no atoms are destroyed
-no atoms are created

Question 10

What is the conservation of mass

Answer 10

-due to no atoms made/gone there are the same number and types of atoms on each side of a reaction equation
-SO no mass is lost or gained (it is conserved)

Question 11

What to do for a question for proof of mass being conserved

Answer 11

1. Add up the relative formula masses on the left hand side of the equation
2. Add up the relative formula masses on the right hand side of the equation
3. they should be equal

Question 12

What to do for a question when finding how many moles are in a mass of a compound

Answer 12

1.Calculate the Mr of the element/compound
2. used the formula for number of moles
-mass should be given

Question 13

What to do for difficult percentage mass questions

Answer 13

1.find the mass of the mentioned material using the percentage
2.calculate its percentage mass of the material in the compound
3. calculate the mass of iron chloride that contains the amount needed

Question 14

What does it mean if mass does change in the reaction

Answer 14

-usually a gas involved
-unsealed reaction vessel

Question 15

What does it mean if the mass increases

Answer 15

-its probably bcs one of the reactants is a gas thats found in air
-the other products are solids, liquids , aqueous

Question 16

What happens before mass increase reaction

Answer 16

-the gas is floating in the air
-its there but not contained in reaction vessel
- so you can’t account for its mass

Question 17

What happens when the gas reacts (mass increase)

Answer 17

-the gas reacts to form part of the product
-it becomes contained inside the reaction vessel
-so the total mass of the stuff inside the reaction vessel increases

Question 18

Example of a reaction with the mass increasing

Answer 18

-when a metal reacts with oxygen in an unsealed container
-the mass of the container increases
-the mass of the metal oxide produced equals the total mass of the metal and the oxygen that reacted from the air

Question 19

What does it mean if the mass decreases due to the reaction

Answer 19

-if the mass decreases its probs bcs one of the products is a gas
-and all the reactants are solids, liquids and aqueous

Question 20

What happens before the reaction (mass decrease)

Answer 20

before the reaction all the reactants are contained in the reaction vessel

Question 21

What happens if the vessel isnt enclosed (mass decrease)

Answer 21

-the gas can escape from the reaction vessel as its formed
-no longer contained so you cant account for its mass
-the total mass of the stuff inside the reaction vessel decreases

Question 22

Example of a reaction with the mass decreasing

Answer 22

-when a metal carbonate thermally decomposes to form a metal oxide and CO2 gas
-if not sealed the mass of the vessel will decrease
-in reality the mass of the metal oxide and Co2 gas equals the mass of the metal carbonate that decomposed

Question 23

What can we use moles for

Answer 23

to calculate masses in reactions

Question 24

How to know how many moles a substance has

Answer 24

-shown in the big number infront of the chemical formulas
-if no number than its one
eg. 2H means 2 moles of hydrogen

Question 25

What to ignore when seeing what the moles are in a substance

Answer 25

ignore if it has any small numbers at the end

Question 26

What do u need to know to work out a balanced symbol equation

Answer 26

masses of the reactants and products

Question 27

How to work out the balanced symbol equation with the masses of reactants and products

Answer 27

1. divide the mass of each substance by its relative formula mass to find the number of moles 2. divide the number of moles of each substance by smallest number of moles in the reaction 3.if any arent whole numbers multiply all numbers 4. write the equation by putting these numbers infront

Question 28

What is a limited reaction

Answer 28

if one reactant gets completely used up in a reaction before the rest then the reaction will stop

Question 29

What do u call the reactant that is used up in a reaction

Answer 29

the limiting reactant

Question 30

What do u call the other reactants that havent been used up

Answer 30

they are in excess

Question 31

Relationship between amount of product formed and limiting reactant

Answer 31

they are directly proportional

Question 32

Why is the amount of product formed directly proportional to the limiting reactant

Answer 32

-if u add more reactant there will be more reactant particles to take part in the reaction -which means more product particles

Question 33

How can u calculate the mass of a product formed in a reaction

Answer 33

by using the mass of the limiting reactant and the balanced reaction equation

Question 34

How to use the limiting reactant and the balanced reaction equation to find the mass of the product

Answer 34

1.write out the balanced equation 2.work out the Mr of the reactant +product u want 3.find out how many moles there are in the known mass 4. use the balanced equation to work out how many moles there will be of the other 5. use the number of moles to calculate the mass

Question 35

What is concentration (casual explain)

Answer 35

the measure of how crowded things are

Question 36

What is concentration (posh)

Answer 36

the amount of a substance (eg.the mass or number of moles) in a certain volume of a solution

Question 37

The more solute in given volume

Answer 37

(the substance thats dissolved) the more concentrated the solution

Question 38

Equation for the concentration

Answer 38

concentration = mass of solute(g) DIVIDE volume of solvent (dm3)

Question 39

How to covert cm3 into dm3

Answer 39

dividing by 1000