Answers to 12 HOMEWORK Sheet 1

YEAR 11 REVIEW & QUALITATIVE/QUANTITATIVE

1. Note that an analysis does not need to be accurate to be quantitative - all there needs to be is some estimate of amount.

(a)A pregnancy test kit is qualitative as we are detecting the presence only

(b)A chlorine test is quantitative as we are estimating the amount.

(c)Placement of a detector in the exhaust pipe of a car is quantitative as we are measuring the amount

(d) Testing for caffeine in a coffee is qualitative as we are detecting the presence only

(e) An analysis of the preservatives in a bottle of apple juice is qualitative as we are detecting the presence of preservatives "202 and 211" only.

(f) Adding a piece of test-tape to urine is quantitative as we are estimating the amount

(g) Instrumental checking of the level of lead in a sample of soil is quantitative as we are measuring the amount

(h) Analysis of a brand of potato chips is quantitative as we are measuring the amount

HCO₃^- (hydrogen carbonate)
Cl^- (chloride)
Mg²⁺ (magnesium)
Na⁺ (sodium)
Ca²⁺ (calcium)
SO₄^2- (sulfate)
K⁺ (potassium)

(b) Which of the ions above are cations?

Mg²⁺ (magnesium)
Na⁺ (sodium)
Ca²⁺ (calcium)
K⁺ (potassium)

(c) Is this information the result of qualitative or quantitative analysis?

Quantitative

(d) Which of the mineral waters would be more beneficial for those on a low salt diet?

Farmland which has only 20 mg/L compared to the Savings which has 53 mg/L

(e) What mass of chloride ions are present in 1.25L of both products?

These calculations are based on the m = c (in g/L) x Vol (L)

mass Cl^- in Savings = 53 x 1.25 = 66.25 mg
mass Cl^- in Farmland = 20 x 1.25 = 25 mg

(f) Determine the mass of each individual ions that would be present in one glass (200 ml) of the "Savings" water.

These calculations are based on the m = c (in g/L) x Vol (L)

mass HCO₃^- in 200 ml water = 508 x 0.200 = 101.6 mg
mass Cl^- in 200 ml water = 53 x 0.200 = 10.6 mg
mass Mg²⁺ in 200 ml water = 43 x 0.200 = 8.6 mg
mass Na⁺ in 200 ml water = 42 x 0.200 = 8.4 mg
mass Ca²⁺ in 200 ml water = 27 x 0.200 = 5.4 mg
mass SO₄^2- in 200 ml water = 16 x 0.200 = 3.2 mg
mass K⁺ in 200 ml water = 5 x 0.200 = 1 mg

(g) Determine the molar concentration of all the anions in the "Savings" water

This is a 2 step process whereby you calculate n using n = m/M (you may need to calculate M). Mass must be expressed in g (ie divide by 1000). Then find the concentration using c = n/V. We use 1L as the ppm concentrations are expressed per L.

For HCO₃^-:
M (HCO₃^-) = 1 + 12 + 3 x 16 = 61
n = 0.508/61 = 0.00833 mol
c = 0.00833/1 = 0.00833 M (or mol/L)

For Cl^-
n (Cl^-) = 0.053/35.5 = 0.0015 mol
c = 0.0015/1 = 0.0015 M (or mol/L)

For SO₄^2-:
M (SO₄^2-) = 32 + 4 x 16 = 96
n = 0.016/96 =0.00017 mol
c = 0.00017/1 = 1.7 x 10^-4 M (or mol/L)

(h) Which species of ions would form a precipitate when mixed with silver nitrate solution?

(i) Sometimes the term "ppm" is used on a label. What does this mean?

This means "Parts per million", ie mg per Kg, or ml per L, or g per tonne etc.

(j) What do the terms v/v, m/v and m/m mean?

These are all expressions of Concentration
v/v = volume solute per volume solvent
m/v = mass solute per volume solvent
m/m = mass solute per mass solvent

(a) False
(b) False - they turn it red
(c) False
d) True - this is the Arrhenius definition of an acid (later replaced by the Bronsted definition of acids being proton donors)
(e) True
(f) False - from the SEP table, Zinc is a stronger reductant than iron as it has a lower E^o value
(g) False
(h) False - Vinegar contains acetic (ethanoic), citric acid is in lemon juice etc.
(i) False - it is insoluble
(j) True
(k) False
(l) False
(m) False - even though CO₂ is non-polar, it reacts with the water in an ionisation reaction.
(n) False - try 2 parts hydrogen to one part oxygen
(o) True
(p) True
(q) False - The atomic number of carbon is 6
(r) True - the Fe is the reductant, the copper (II) ion acts as the oxidant
(s) False
(t) True
(u) True
(v) True
(w) True - the 3rd shell continues to fill after 2 electrons are placed in the 4th shell
(x) False - Oxygen wants 2 electrons
(y) True - the main isotopes of K have less neutrons and protons (nucleons) than those of Ar
(z) False - it's pink in bases

So how did you fare this week?