Chemistry Solutions 9701/12/M/J/15

Ref: 9701/12/M/J/15 #2
The shell of a chicken's egg makes up 5% of the mass of an average egg. An average egg has a mass of 50g.
Assume the egg shell is pure calcium carbonate.
How many complete chicken's egg shells would be needed to neutralise 50cm3 of 2.0moldm-3 ethanoic acid?
A. 1
B. 2
C. 3
D. 4


Solution
B 2
Form the balanced equation for the reaction of ethanoic acid with calcium carbonate.
2CH3COOH+CaCO3-->(CH3COO)2Ca+CO2+H2O
For an average egg, the shell is 0.05 x 50=2.5g. The Mr of CaCO3 is 100.1. Hence a chicken's egg shell contains 2.5/100.1=0.025 mol of CaCO3. 50cm3 of 2.0moldm-3= 50 x 2/1000=0.1mol of ethanoic acid. From the balanced equation, 2 mols of ethanoic acid reacts with 1 mol of CaCO3. Hence, 0.05 mols of calcium carbonate would be required to react with the amount of ethanoic acid stated in the question.
Therefore, number of chicken's egg shells=0.-05/0.025= 2.


Ref: 9701/12/M/J/15 #9
When a sample of HI is warmed to a particular temperature the equilibrium below is established.

2HI(g)--->< H2 (g)+ I2 (g)

At this temperature, it is found that the partial pressure of HI(g) is 28 times the partial pressure of H2 (g).

What is the value of Kp at this temperature?

A. 1.28 X 10-3

B. 0.035

C. 28

D. 784

Solution

A 1.28 x 10-3

Let the partial pressure of H2 be x. Since there are the same number of moles of H2 and I2, the partial pressure of I2 is x as well from the decomposition of HI. Hence the partial pressure of HI is 28x. Kp=x^2/(28x)^2= x^2/784x^2. By cancelling out x^2, Kp= 1/784= 1.28 x 10-3.

Ref: 9701/12/M/J/15 #10
Photochromic glass, used for sunglasses, darkens when exposed to bright light and becomes more transparent again when the light is less bright. The darkness of the glass is related to the concentration of silver atoms.
The following reactions are involved.

reaction 1 Ag+ + Cl- --->< Ag+ Cl

reaction 2  Cu+ +Cl---> Cu2+ +Cl-

   reaction 3 Cu2+ +Ag---> Cu+ + Ag+



Which statement about these reactions is correct?
A. Cu+ and Cu2+ ions act as catalysts.
B. Cu+ ions act as an oxidising agent in reaction 2.
C. Reaction 3 increases the darkness of the glass.
D. Silver atoms are reduced in reaction 3.


Solution
A Cu+ and Cu2+ ions act as catalysts.
Statement 1 is correct as the Cu2+ and Cu+ ions are regenerated at the end of reactions 2 and 3 respectively, hence they act as catalysts.
Statement 2 is incorrect as the oxidation number of Cl changes from 0 to -1, hence the Cu+ ions act as a reducing agent.
Statement 3 is incorrect as silver atoms are converted to silver ions. Silver atoms increase the darkness of the glass whereas silver ions cause the glass to become more transparent.
Statement 4 is incorrect as the oxidation number of silver increases from 0 to +1, hence the silver atoms are oxidised.


Ref: 9701/12/M/J/15 #12
Use of the Data Booklet is relevant to this question.
When 3.00 g of an anhydrous nitrate of a Group II metal is decomposed, 1.53g of gas is produced. What is the nitrate compound?
A. beryllium nitrate
B. calcium nitrate
C. magnesium nitrate
D. strontium nitrate


Solution
D strontium nitrate
The decomposition of anhydrous Group II nitrates will produce the metal oxide, nitrogen dioxide gas and oxygen gas. The balanced equation for this process is

2X(NO3)2--->2XO+4NO2+O2



*X represents the Group II metal
There are two types of gases produced. Hence it is not possible to find the number of moles of the gases produced with the information given. So we must calculate the amount of solid metal oxide produced, which is 3.00--1.53=1.47g.
From the balanced equation, it can be seen that 2 mols of the metal nitrate will produce 2 mols of the metal oxide. Hence it is a 1:1 mole ratio.
To solve this question, the Mr of each metal nitrate and metal oxide must be calculated. The answer would be the compound which has equal number of moles of metal nitrate and metal oxide for the masses given.
For this question, the number of moles of strontium nitrate= 3.00/211.6=0.01418 mol. The number of moles of strontium oxide=1.47/103.6=0.01419 mol. Thus the answer is D.


Ref: 9701/12/M/J/15 #30
The drug Sirolimus is used to treat patients after kidney transplants.

On reaction with hot aqueous sodium hydroxide, Sirolimus produces an equimolar mixture of two organic products.
What is the structural formula of the product with the lower relative molecular mass?

Solution
B

At the top left corner, there is an ester bond and an amide bond. When heated with hot aqueous sodium hydroxide, the compound undergoes alkaline hydrolysis of amide and ester and the two bonds break, producing two compounds. The compound with the lower relative molecular mass contains the carboxylic part of the ester (COO- group), forming a salt with the sodium ion from NaOH, thus producing CO2Na. This compound also has a secondary amine group from the hydrolysis of the amide bond. So the answer is B.

Ref: 9701/12/M/J/15 #33
Which statements are correct for all exothermic reactions?
1 H for the reaction is negative
2 On a reaction pathway diagram the products are shown lower than the reactants.
3 The reaction will happen spontaneously


Solution
B 1 and 2 only
Statement 3 is incorrect as the enthalpy change of a reaction does not determine its rate or whether it occurs. An example is the conversion of diamond to graphite which is an exothermic reaction, but occurs at an immeasurably slow rate due to the large activation energy for the reaction.
Statements 1 and 2 are true for all exothermic reactions, so the answer is B.