topic 3 recap Q. chemical calculations

Question 1

what is the conservation of mass?

Answer 1

atoms are not created or destroyed so total mass before = total mass after

Question 2

when a metal forms a metal oxide, why does the mass increase?

Answer 2

atoms from gaseous oxygen have been added from the air?

Question 3

what is relative formula mass?

Answer 3

the sum of the relative atomic masses of each atom in a substance

Question 3

when an acid reacts with a metal why does the mass decrease?

Answer 3

a gas is produced and escapes into the air?

Question 4

what are the 4 state symbols?

Answer 4

(s) solid
(l) liquid
(g) gas
(aq) dissolved in water

Question 5

if the amount of solute in a solution is increased what happens to the concentration?

Answer 5

increases

Question 6

what is a mole?

Answer 6

mass of a substance that contains 6.02 x 1023 paricles

Question 7

how can you tell when a symbol equation is balanced?

Answer 7

the number of atoms of each element is the same on both sides.

Question 8

which formula is used to calculate the number of moles from mass and Mr?

Answer 8

moles = mass/Mr

Question 9

give the value for avagadros constant

Answer 9

6.02 x 10 23

Question 10

what is the unit for concentration?

Answer 10

g/dm3 or mol/dm3

Question 10

which formula is used to calculate the mass of a substance from number of moles and Mr?

Answer 10

mass= moles x Mr

Question 11

if the volume of water in a solution increased what happens to the concentration?

Answer 11

decreases

Question 12

what is a limiting reactant?

Answer 12

the reactant that is completely used up in a chemical reaction

Question 13

which formula is used to calculate volume from concentration and mass?

Answer 13

volume (dm3) = mass (g) / concentration (g/dm3)

Question 14

which formula is used to calculate concentration from mass and volume?

Answer 14

concentration (g/dm3) = mass (g) / volume(dm3)

Question 15

how can you convert a volume reading in cm3 to dm3?

Answer 15

divide by 1000

Question 15

which formula is used to calculate mass from concentration in g/dm3 and volume?

Answer 15

mass (g) = concentration (g/dm3) x volume (dm3)

Question 16

What is the yield of a reaction?

Answer 16

mass of product obtained from the reaction

Question 17

What is the theoretical yield of a reaction?

Answer 17

maximum mass of the product that could have been produced

Question 18

Why is the actual yield always less than the theoretical yield?

Answer 18

• reaction may be reversible
• some of the product can be lost on separation
• unexpected side reactions between reactants

Question 19

What is the percentage yield?

Answer 19

actual yield as a proportion of theoretical yield

Question 20

How is percentage yield calculated?

Answer 20

actual yield divided by theoretical yield x 100

Question 21

Why is a high atom economy desirable?

Answer 21

results in less waste/is more sustainable

Question 22

What is atom economy?

Answer 22

measure of how many atoms of the reactants end up as useful products

Question 23

How is percentage atom economy calculated?

Answer 23

(Mr of useful product/Mr of all products) × 100

Question 24

How can concentration in mol/dm3 be calculated?

Answer 24

concentration (mol/dm3) = moles (mol) / volume (dm3)

Question 25

What is a titration?

Answer 25

method used to calculate the concentration of an unknown solution

Question 26

What is the end-point?

Answer 26

the point at which the reaction is complete (when the indicator changes colour) and no substance is in excess

Question 27

How should solution be added from the burette close to the end point?

Answer 27

drop by drop, swirling in between

Question 28

Why is a white tile used in titration?

Answer 28

to see the colour change better

Question 29

What is a titre?

Answer 29

volume of solution added from the burette

Question 30

What are concordant titres?

Answer 30

titres within 0.1 cm3 of each other

Question 31

What volume does one mole of any gas occupy at room temperature and pressure?

Answer 31

24 dm3 or 24 000 cm3