11 Chemistry - Stoichiometry

Question 1

Define: mole / avogadro’s constant

Answer 1

(6.02X10^23) things

Question 2

How many molecules 1 mole of hydrogen gas has?

Answer 2

(6.02X10^23)

Question 3

How many moles is:

a) 3.01 X 10^23 =
b) 18 X 10^22 =

Answer 3

a) 0.5 moles
b) 0.299 moles

how many
_________ = moles
avogadro

Question 4

How many is:

a)2.8 moles of h2?

Answer 4

a) 1.6856 x 10^24

moles x avogadro = how many

Question 5

If I have 0.5 moles of CuSO4

a) how many molecules of copper sulphate are there?
b) how many oxygen atoms are there?

Answer 5

a) 0.5(avogadros) = 3.01x10^23

b) (3.01x10^23) x 4 = 1.204 x 10^24

Question 6

What is the atomic mass (Ar) of an atom equal to?

Answer 6

It is equal to the mass on grams of one mole of that element

eg. copper, Cu, has an atomic mass of 64, this means one mole of Cu is 64g

Question 7

What is the molecular mass (Mr) of a compound equal to?

Answer 7

It is equal to the mass of one mole of a compound

eg. NaCl Na = 23 Cl = 33.5
so NaCl = 23+33.5
= 58.5g is the mass of one mole of NaCl

Question 8

Find the mass of one mole of:

a) methane CH4
b) Ethanic acid CH3COOH
c) Glucose

Answer 8

a) 16g
b) 60g
c) 180g

Question 9

Equation for moles and mass

Answer 9

mass mass
Moles = _______ moles formula mass
Ar or Mr

Question 10

What is concentration?

Answer 10

The amount of solute in a given volume of solution

Question 11

What is the unit for concentration?

Answer 11

mol dm^-3 or (mol/dm^3)

Question 12

What is the equation for concentration?

Answer 12

moles                           vol(dm^3)
concentration  =  \_\_\_\_\_\_\_\_                  moles  24dm^3
                             volume(dm^3)

Question 13

How much is 1 dm^3?

Answer 13

1 litre

Question 14

At room temp, what is the volume of 1 mole of any gas?

Answer 14

24dm^3

Question 15

At 0°C, what is the volume of 1 mole of any gas?

Answer 15

22.7dm^3

Question 16

Equation for moles in gases

Answer 16

volume volume
moles of gas = ________ moles 24dm^3
24dm^3

Question 17

equation of combustion

Answer 17

something + 2oxygen –> carbon dioxide + water

Question 18

If i burnt 10g of methane, what mass of oxygen would be needed?

Answer 18

1) 10/16 = 0.625 moles of methane

2) We know from the combustion equation that every 1 molecule of methane reacts with 2 of oxygen
so, 0.625 x 2 = 1.25 moles of oxygen

3) moles x Mr = mass
1. 25 x 32 = 40g of oxygen

Question 19

What are all equations that link moles, to mass, to gas volume, to concentration?

Answer 19

mass volume moles

moles Mr moles 24dm^3 concentration 24dm^3

Question 20

What does an acid + base give?

Give an example with hydrochloric acid and sodium hydroxide

Answer 20

acid + base = neutralisation

hydrochloric acid + sodium hydroxide = water + salt

Question 21

What is the relative atomic mass (Ar)?

Answer 21

the weighted average of all isotopes of an element, compared to carbon 12

Question 22

What is relative formula mass (Mr)?

Answer 22

the combined mass of the atoms in a compound, compared to carbon 12

Question 23

What do these symbols mean?

a) (g)
b) (s)
c) (aq)
d) (l)

Answer 23

a) (g) gas
b) (s) solid
c) (aq) aqueous - dissolved in solution
d) (l) liquid

Question 24

What does limiting reagent and excess reagent mean?

Answer 24

Limiting - the one you have less of, the one that is all used up, how much is produced depends on it

Excess - the one you have more of, still some left at then end but in smaller amount

Question 25

What are the ionic charges of metals?

Answer 25

always +ive
group number (except transition metals)

eg. Na+ Al3+

Question 26

What are the ionic charges of non-metals?

Answer 26

always -ve
charge = 8 - goup number

eg. O2- Cl-

Question 27

What are the charges for these polyatomic anions?

a) CO3
b) SO4
c) OH
d) NO3
e) PO4

f)NH4 (cation)

Answer 27

a) CO3 2-
b) SO4 2-
c) OH -
d) NO3 -
e) PO4 3-

f)NH4 (cation) +

Question 28

What is empirical formula

Answer 28

simplest ratio of the atoms in a compound

Question 29

what is molecular formula

Answer 29

actual ratio of atoms

eg. C6 H12 O6

Question 30

How do you determine empirical formula?

Answer 30

1- start with no. in grams
if it is % the total mass is 100g, so then the masses = %

2-convert each mass into moles

3-divide each mole by the smallest no. of moles calculated

4-round to the nearest whole number, this is the ratio

Question 31

how do you convert empirical formula to molecular?

Answer 31

1- calculare mass of empirical formula

2- divide molar mass of compound by mass of empirical formula to find ratio

3- multiply all atoms by this ratio

done

Question 32

how do you find the relative atomic mass from percent abundance?

Answer 32

1- each exact atomic weight is multiplied by the percent abundance (as decimal, e.g. 23.2% = 0.232)

2- Add all results together

eg Carbon

Mass no. = 12
Exact weight = 12.000000
% abundance = 98.90

Mass no. = 13
Exact weight = 1.000000
% abundance = 1.10

solution:
(12.000000) (0.9890) + (1.000000) (0.0110) = 12amu

Question 33

Boron has an atomic mass f 10.18amu
however no single boron atom has a mass of 10.10 amu
how do you explain this?

Answer 33

10.18 is the average mass

some atoms have less and some have more but the average turns out to be 10.18