Science - Chemistry - Quantitative Chemistry - Amounts Of Substances- C3a

Question 1

what is the relative formula mass ?

Answer 1

the relative formula mass (Mr) of a compound is all of the relative atomic masses (Ar) of the atoms in that compound added together

Question 2

what is the percentage mass of an element in a compound ?

Answer 2

what proportion of the mass of the compound is due to atoms of that element

Question 3

how do you calculate the percentage mass of an element in a compound ?

Answer 3

Ar x number of atoms of that element/Mr of the compound X100

Question 4

what is the Avogadro constant ?

Answer 4

a number -
602 000 000 000 000 000 000 000

Question 5

what is a mole ?

Answer 5

a unit of measurement -
1 mole of any substance is just the amount of that substance that contains an Avogadro number of particles, so 6.02 x 10 to the power of 23 particles.

Question 6

what is the mass of one mole equal to ?

Answer 6

the same number of grams as the relative atomic mass (Ar) or relative formula mass (Mr) of the element or compound

Question 7

how do you calculate the number of moles ?

Answer 7

mass in g ( of element or compound) / Mr ( of element or compound) or Ar (of element)

Question 8

what is the conservation of mass ?

Answer 8

in a chemical reaction, no atoms are made or destroyed, so the mass of the products is the same as the mass of reactants

Question 9

how can you prove the conservation of mass ?

Answer 9

by adding the relative formula masses of the substances on each side of the balanced symbol equation, total Mr of reactants = total Mr of products

Question 10

why does the mass seem to change in some reactions ?

Answer 10

in some reactions, where the container isn’t sealed the mass may increase or decrease depending on whether there’s a state change

Question 11

why does the mass increase in some reactions ?

Answer 11

• because one of the reactants is a gas that’s found in air, and all the products are solids, liquids or aqueous
• the particles in a gas move around and fill the space they’re in, and before the reaction the gas is floating in the air, it’s there but not contained in the reaction vessel so you can’t account for its mass
• when the gas reacts to form part of the product, the particles become contained inside the reaction vessel, so that the total mass of the stuff inside the reaction vessel increases

Question 12

why does the mass decrease in some reactions ?

Answer 12

• if the mass decreases, it’s probably because one of the products is a gas, and all reactants are solids, liquids or aqueous
• before the reaction, all the reactants are contained in the reaction vessel
• if the vessel isn’t enclosed, the gas that’s produced can escape from the reaction vessel as it’s formed

Question 13

in a balanced equation, what do the big numbers in front of the chemical formulas show us ?

Answer 13

the relative number of moles of each reactant that take part in the reaction, and the relative number of moles of each product that are formed

Question 14

what are the little numbers within the chemical formulas tell you ?

Answer 14

how many atoms of each element there are in the smallest unit of the substance

Question 15

what are the steps in balancing reacting masses ?

Answer 15

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 the smallest number of moles in the reaction
3. if any of the numbers aren’t whole numbers, multiply all the numbers by the same amount so that they all become whole numbers
4. write the balanced symbol equation for the reaction by putting these numbers in front of the chemical formulas

Question 16

what is the limiting reactant ?

Answer 16

the reactant that’s used up in a reaction first, as it limits the amount of product that’s formed, other reactants are in excess

Question 17

what is the amount of product formed directly proportional to ?

Answer 17

the amount of limiting reactant, e.g, if you halve the amount of limiting the amount of product formed will halve

Question 18

how do you calculate the mass of a product formed in a reaction by using the mass of the limiting reactant and the balanced reaction equation ?

Answer 18

1. write out the balanced equation
2. work out the relative formula masses of the limiting reactant and the product you want to find the mass of
3. work out how many moles there are of the limiting reactant
4. use the balanced equation to work out how many moles they’ll be of the product
5. use the number of moles, along with the relative formula mass of the product to calculate the mass

Question 19

when will equal numbers of moles of any gas occupy the same volume ?

Answer 19

they have the same :
- temperature
- pressure

Question 20

how do you calculate the volume of gas (dm cubed)

Answer 20

mass of gas / Mr of gas X24

Question 21

how do you find volumes of gases from equations ?

Answer 21

use the volume of one gas to find the volume of another, just look at the molar ratio of the gases in the balanced equation - this is the same as the ratio of the volumes of the gases in the reaction