Chem1262 2019 - Assessed Assignment 1

CHEM1262 2019 - Assessed Assignment 1
Name................................................................
Student Number....................................................
Q1) Phosphoric acid (H3PO4 - or additive E338) is used to acidify foods and
beverages, a common example being cola drinks. However, the amount present must
be strictly controlled to avoid foods tasting too bitter.
How many moles of phosphoric acid are contained in 15.0 ml of 0.50 M solution of
H3PO4?
(1 mark)
Concentration H2 PO4 =0.50 mol /L
That is 0.50 mol=1000 ml
We now formulate our comparison as follows:
If 1000 ml=0.50 mol
15.0 ml=?
¿ 15.0 ml∗0.50 mol
1000 ml
¿ 0.0075 mol
Q2) Another commonly encountered chemical in foods and beverages is ethanol,
again too much is bad for you.
Calculate the molar concentration of ethanol in a wine that contains 8.0 g of C2H5
OH in 700 L of solution.
(2 marks)
Molar mass ¿ 46 g /mol
This means 46 g=1 mol
8.0 g=?
¿ 8.0 g∗1 mol
46 g =0.174 mol
1

Hence molar concentration ¿ 0.174
400
¿ 0.000248 M
Q3) One of the most common food additives is table salt (NaCl). Health
recommendations advise people to consume no more than 6g of salt per day.
What weight of Cl- (At. Mass = 35.5) is contained in 6.0 g of NaCl (Mol Mass =
58.5)?
Hint you can calculate this using by first working out the percentage composition of
chloride in sodium chloride if you wish.
(2 marks)
58.5 g NaCl contains 35.5 g Cl- (John Kotz, 2008)
1 g NaCl contains 35.5
58.8 g C l−¿¿
6 g NaCl contains 6∗35.5
58.5 g
¿ 3.641 g C l−¿¿
Q4) Lead compounds have been produced for coloured pigments in paints for
hundreds of years. However, lead is toxic, and categorised as probably carcinogenic
to humans by the International Agency for Research on Cancer so it’s applications
are limited and it is important to be careful regarding exposure.
What mass of lead acetate (of molar mass 325.29 g/mol) should be dissolved in 250
mL of water to make a solution of concentration 0.05M?
(2 marks)
Molar mass of led acetate ¿ 325.29 g /mol
This means that 325.29 g=1 mol
Solution concentration to be 0.05M
This means 0.05 moles=1000 ml CITATION Hir11 \l 2057 (Ohno, 2011)
?=250 ml
?= 0.05 moles∗250 ml
1000 ml =0.0125 moles
But for lead acetate, 1 mol=325.29 g
0.0125 moles=?
¿ 0.0125 mol∗325.29 g
1
¿ 4.066 g
2

Q5) Sodium hydroxide (also known as lye or caustic soda) is used as a strong
chemical base in many industries including the manufacture of pulp and paper,
textiles, drinking water, soaps and detergents and as a drain cleaner. Worldwide use
is around 70-80 million, dry metric tonnes per year.
How many grams of sodium hydroxide are dissolved in 1.0 L of solution of sodium
hydroxide (formula NaOH, molecular mass = 40) with a concentration of 0.5M.
(2 marks)
Molar mass of NaOH = 40 g/mol
In other terms, 40 g=1 mol
Concentration ¿ 0.5 M
This means that 0.5 moles are contained in 1.0 L of the solution (Mark M. Benjamin,
2013).
Thus, the mass of NaOH in 1.0 L = 0.5∗40
1 =20 g
Q6) A good chemist will always acknowledge that errors occur within any data set,
however carefully they perform the analysis. It is therefore good practice to perform
analyses several times in order that some certainty may be given that the test gives a
true and valid reading.
Two terms related to this repetition of testing are precision and accuracy. Define
each.
(2 marks)
Precision is the closeness of two or more measurements or results to each other.
Accuracy is the closeness of a result or measurements to the true value.
Q7) The following data were obtained from replicate measurements of the
determination of lead ion (Pb2+) in parts per million (or milligrams per liter) in a
sample of water which was suspected of being contaminated from lead from a
nearby disused mine.
Determine (a) the mean, (b) the standard deviation, (c) the relative standard
deviation expressed as a percentage, (d) the spread and (e) the range error for the
data.
3
Replicate 1 2 3 4 5 6 7
Pb2+ 20.1 19.5 20.3 19.7 20.0 19.4 19.6