UOIT Chemistry CHEM 1010U First midtem test sotlutions. Fall 2009

1. Choose four of the following terms. Concisely explain what each one means in a few

sentences.

Covalent bond

Isotope

Percent yield

Titration

Redox reaction

Solute

TC

Ionic interactions

A mole

Limiting reagent

Yield

Solvent

TD

Molecule

4 Marks each

A covalent bond is a shared electron pair between two atoms (not elements!). Covalent bonds

point is specific directions to give molecules their shapes.

Ionic interactions are governed by the Coulomb law, which states that ions with like charge will

repel each other, and those with opposite charge will attract each other. Unlike covalent bonds,

ionic interactions don’t have a direction. (Note that the transfer of electrons from one atom to

another describes a redox process).

Isotopes of an element have the same atomic number (number of protons) but have different

numbers of neutrons. Hence they have different mass numbers.

A mole is an Avogadro’s number of things.

Percent yield is the yield over the theoretical yield, times 100%. It is typically less than 100%, as

side or incomplete reactions reduce the yield.

The limiting reagent is the reactant that is fully consumed before the other reactants. Once it

runs out, the reaction stops and no more product can be made. The amount of the limiting

reagent determines the theoretical yield.

A titration is an analytical process where an analyte is reacted with a titrant (not an acid and a

base!). The moles of analyte present can be found, since at the equivalence point the titrant

fully reacts with the analyte, according to the stoichiometric ratio of the titration reaction.

The yield is the amount product produced. Can be measured in moles or grams.

In a redox reaction one atom undergoes reduction (gains electrons) and another is oxidized

(loses electrons).

The solvent is the liquid in which solutes are dissolved to make a solution.

Solute is the chemical (often a solid) that is dissolved in a solvent to make a solution.

TD means “to deliver”. This kind of glassware is designed to pour out the indicated volume of

liquid.

TC means “to contain”. This kind of glassware is designed to hold the indicated volume of liquid.

A molecule is a given set of atoms (not elements!) held together in a specific shape by covalent

bonds.

2. Using the periodic table to the right, answer the

following questions.

a) In what way are elements A and B related?

Elements A and B are in the same group. Because of this,

we expect them to have similar chemical properties.

4 Marks

b) In what way are elements B and E related?

B and E are in the same period.

2 Marks

c) Pick an element (A, B, C, D or E). Thinking about the position of the element in the periodic

table, list its properties.

Element C is in group 17, and is a halogen. These elements are very reactive non-metals, and

can for diatomic molecules with themselves.

4 Marks

Total 10 Marks

3. A typical mosquito has a mass of 1.5 mg. If the mosquito’s feet have a total surface area of

0.05 mm2, what pressure does it exert if it stood on your arm at the moon? Express your

answer using the fundamental SI units. Pressure, by definition, is force per unit area. Force is

given by mass times acceleration. The acceleration due to the Moon’s gravity is 1.6 m/s2. Do

not worry about significant figures for this question.

Total 10 Marks

� ��

�, � � ��, � �

��

�

� �1.5 � 10�� kg � 9.8 m

s2�

0.05 � 10�� m2 � 294 kgm s2�

or,

� �1.5 � 10�� kg � 1.6 m

s2�

0.05 � 10�� m2 � 48 kgm s2�

4. Two isotopes of tin are 117

Sn and 119

Sn.

a) What is the atomic number of 119

Sn?

50

2 Marks

b) How many neutrons does 117

Sn have?

117 - 50 = 67

2 Marks

c) Do you expect 117

Sn and 119

Sn to have similar chemical properties? Explain why or why not.

Since these are isotopes of the same element, they should have the same chemical properties.

6 Marks

Total 8 Marks

4. Two isotopes of hydrogen are 2H (deuterium) and

3H (tritium).

a) What is the atomic number of deuterium?

1

2 Marks

b) What is the mass number of tritium?

3

2 Marks

c) Do you expect deuterium and tritium to have similar chemical properties? Explain why or

why not.

Since these are isotopes of the same element, they should have the same chemical properties.

6 Marks

Total 8 Marks

5. Name the following chemicals.

a) SnCl4 b) HI c) N2O4

a) Tin (IV) chloride b) Hydrogen iodide, or, c) dinitrogen tetroxide

hydroiodic acid

Total 6 Marks

5. Name the following chemicals.

a) SnCl2 b) HBr c) N2O4

a) Tin (II) chloride b) Hydrogen bromide, or, c) dinitrogen tetroxide

hyrdobromic acid

Total 6 Marks

6. What are the chemical formulae for the following?

a) Iron (III) nitrate b) Sodium perchlorate c) Dihydrogen dioxide

a) Fe(NO3)3 b) NaClO4 c) H2O2

6. What are the chemical formulae for the following?

a) Iron (III) sulfate b) Potassium perchlorate c) Dihydrogen monoxide

a) Fe2(SO4)3 b) KClO4 c) H2O

7) You are asked to make 0.5000 litre of a 1.100 M stock solution of NaOH. How many grams of

NaOH (s) would you need?

4 Marks

�NaOH � � � � 0.5000 L � 1.100 M � 0.5500 mol

%%NaOH � 22.99 amu ' 16.00 amu ' 1.008 amu � 39.998 amu � 39.998 gmol�

�NaOH � � � %% � 0.5500 mol � 39.998 gmol� � 22.00 g

You pipette 25.00 mL of the stock solution into a 250 mL volumetric flask, and dilute to the

mark. What is the concentration of the diluted solution?

�NaOH � � � � 0.02500 L � 1.100 M � 0.02750 mol

*NaOH � � + � � 0.02750 mol � 0.250 L � 0.110 M

4 Marks

How many grams of phosphoric acid (H3PO4) would 10.00 mL of your diluted NaOH solution

react with? This neutralization reaction goes to 100% completion.

In 10.00 mL, the moles of NaOH is

�NaOH � � � � 0.01000 L + 0.110 M � 1.10 � 10�,mol.

Since phosphoric acid is triprotic, one mole of NaOH will neutralize ⅓ mole of the acid.

Therefore,

�H3PO4�

�NaOH

3�

1.10 � 10�,mol

3� 3.66� � 10�.mol

%%H3PO4� 3 � 1.008 amu ' 30.97 amu ' 4 � 16.00 amu � 97.994 g

mol�

�H3PO4� � � %% � 3.66� � 10�.mol � 97.994 g

mol� � 0.0359 g

12 Marks

Total 20 Marks

8. It takes 36.43 mL of a 0.2563 M EDTA solution to titrate 25.00 mL of an iron (II) solution to

the end point. The titration reaction is

Fe2+

+ EDTA ↓ Fe-EDTA2+

.

a) What is the titrant? What is the analyte?

The EDTA solution is the titrant, and the iron is the analyte.

4 Marks

b) What is the concentration of the iron (II) solution?

From the titration reaction, we see that the iron and EDTA react 1:1. Therefore, at the

equivalence point, the moles of EDTA added are equal to the moles of Fe2+

.

�EDTA � � � � 0.3643 L � 0.2563 M � 0.009337 mol � �Fe56

Fe56 � � + � � 0.009337 mol + 0.03643 L � 0.3735 M

16 Marks

Total 20 Marks

9. For the following redox reaction,

Sn2+

(aq) + IO4- (aq) ↓ Sn

4+ (aq) + I

- (aq)

a) Identify which species are oxidized and reduced.

The oxidation numbers of the atoms are:

Sn2+

(aq) : tin has an oxidation number of +2

IO4- (aq) : the polyatomic ion has a charge of 1-, therefore the sum of the oxidation numbers

has to be -1. Each oxygen has an oxidation number of -2, and there are four of them, for a total

of -8. Therefore the oxidation number of iodine must be +7.

Sn4+

(aq) : tin has an oxidation number of +4

I- (aq) : iodine has an oxidation number of -1.

Tin’s oxidation number goes up (from +2 to +4), tin (or Sn2+

) is oxidized.

Iodine’s oxidation number goes down (from +7 to -1). Iodine is reduced (not IO4-).

4 Marks

b) Balance the reaction in acidic solution.

The oxidation produces two electrons per tin atom, the reduction requires eight electrons per

iodine atom. Therefore four tin atoms must be oxidized for each iodine that is reduced.

4 Sn2+

(aq) + IO4- (aq) ↓ 4 Sn

4+ (aq) + I

- (aq)

There is an abundance of oxygen on the reactants’ side. Add water to balance the oxygen. Need

four waters to provide four oxygens.

4 Sn2+

(aq) + IO4- (aq) ↓ 4 Sn

4+ (aq) + I

- (aq) + 4 H2O

There is now an excess (eight) of hydrogen on the products’ side. Add H+ (acidic solution) on the

other side to balance.

4 Sn2+

(aq) + IO4- (aq) + 8 H

+ (aq) ↓ 4 Sn

4+ (aq) + I

- (aq) + 4 H2O

Check:

Sn – four on each side

I – one on each side

O – four on each side

H – eight on each side

charge: 15+ on each side. The reaction is balanced.

8 Marks

Total 12 Marks

10. When 23.46 mg of ethylene glycol is burned, it yields 20.42 mg of H2O and 33.27 mg of CO2.

a) What is the empirical formula of ethylene glycol?

All the carbon in the fuel winds up in the CO2. The molar mass of CO2 is 44.01 g/mol.

�CO2� � + %% � 0.03327 g + 44.01 g

mol� � 7.559� � 10�.mol

There must have been 7.559 × 10-4

mol C in the fuel.

All the hydrogen in the fuel winds up in the water. The molar mass of water is 18.016 g/mol

�H2O � � + %% � 0.02042 g + 18.016 gmol� � 1.133. � 10�,mol

Each water molecule contains two hydrogen atoms. Therefore, there must have been

1.133 × 10-3

× 2 = 2.266 × 10-3

mol H in the fuel.

The mass of C in the fuel is

�C � �C � %% � 7.559� � 10�. mol � 12.00 gmol� � 0.009072 g

The mass of H in the fuel is

�H � �H � %% � 2.266 � 10�, mol � 1.008 gmol� � 0.002285 g

The total mass of C and H in the fuel is 0.009072 g + 0.002285 g = 0.01136 g. The rest must be

oxygen.

The mass of oxygen is the total mass of the fuel minus the mass of hydrogen and carbon.

0.02346 g – 0.01136 g = 0.01210 g

The molar mass of oxygen is 16.00 g/mol. Therefore the moles of oxygen is

�O � � + %% � 0.01210 g + 16.00 gmol� � 7.563 � 10�.mol

The empirical formula is the mole ratio of the elements that make up a substance.

nC : nH : nO = 7.559 × 10-4

: 2.266× 10-3

: 7.563 × 10-4

Diving each by the smallest number of moles,

nC : nH : nO ≈ 1 : 3 : 1

Therefore the empirical formula is CH3O.

8 Marks

b) If the molecular mass of ethylene glycol is 62.0 amu, what is the molecular formula?

The mass of the empirical formula is 12 + 3 × 1 + 16 = 31 amu. Therefore the molecule is twice

as heavy as the empirical formula.

Therefore, the molecular formula is C2H6O2.

4 Marks

Total 12 Marks

Exam total 120 Marks