Question 43 : [Physical > Atoms, Molecules & Stoichiometry]
Reference: Past Exam Paper -June 2003 Paper Q3
Solution 43:
Answer: D.
sulphur number of neutrons = 32-16 = 16.
A cannot be answer because number of neutrons of Na = 23-11 = 12
B cannot be answer because number of neutrons of Mg = 24-12 = 12
C cannot be answer because number of neutrons of Si = 28-14 = 14
D is the answer because number of neutrons of P = 31-15 = 16
therefore contains the same number of neutrons as S.
Question 42 : [Physical > Atoms, Molecules & Stoichiometry]
In the preparation of soft margarine, glyceryl trieleostearate
A 4
B 5
C 6
D 9
Reference: Past Exam Paper -June 2003 Paper 1 Q2
Solution 42:
Answer: B.
Glyceryl trieleostearate contains 3 chains . in the question its mentioned that the compound is hydrogenated therefore we are expecting hydrogen molecules to be added to the compound. in each of the compound’s chain there are 3 double bonds. and the question already told us that one of the side chains was converted to a new side chain which has now 2 double bonds instead of 3 this means that one hydrogen molecules was added to one side chain by electrophilic addition. the other 2 side chains are said to be converted to new side chains , in each of the 2 other side chains it said now it contains 1 double bond instead of 3 , this means in each side chain of the 2 remaining chains have 2 hydrogen molecules added to it. therefore one hydrogen molecules was added to one side chain. 2 hydrogen molecules were added to each of the two remaining side chains , this therefore make a total of 5 hydrogen molecules. therefore 5 moles of hydrogen since each hydrogen molecule contains 2 hydrogen atoms.
Question 41 : [Physical > Atoms, Molecules & Stoichiometry]
The use of the Data Booklet is relevant to this question.
What is the number of molecules in 500 cm3 of oxygen under room conditions?
A 1.25 x 10 22
B 1.34 x 10 22
C 3.0 x 10 22
D 3.0 x 10 26
Reference: Past Exam Paper - June 2003 Paper 1 Q1
Solution 41:
Answer: A.
Solution : since its room conditions 1 mole of any gas occupy 24dm3 therefore we can use the formula volume= no.of moles x 24dm3. since we are given the volume of oxygen lets find the no.of moles . but before proceeding its easy to convert the volume that is given in cm3 to dm3 , therefore we now calculate like this : no.of moles : 500x10 -23 / 24 = 0.0208.
now we got the number of moles its easy to find the number of molecules or atoms , we can apply Avogadro constant , since its a gas , 02 then the number of moles we got which we found it to be 0.0208 x 6.02 x 10 23 = 1.25 x 10 22. if we found the number of moles and we applied Avogadro constant for diatomic molecules such as oxygen we get the number of molecules if we want to find the number of atoms we multiply the number of molecules we got by 2 . but for single atomic elements such as K , Na , we can apply the Avogadro constant to get the no. of atoms directly.
Question 1: [Physical
> Atoms, Molecules & Stoichiometry]
A mixture of 10 cm3of methane and 10 cm3of
ethane was sparked with an excess of oxygen.
After cooling to room temperature, the residual gas was
passed through aqueous potassium hydroxide.
What volume of gas was absorbed by the alkali?
A 15 cm3
B 20 cm3
C 30 cm3
D 40 cm3
Reference: Past Exam Paper –November 2002 Paper 1 Q1
Solution 1:
Answer: C.
Methane and ethane are both hydrocarbons. When they undergo
combustion they produce carbon dioxide and water. These products were cooled to
room temperature. Carbon dioxide remains a gas and water becomes a liquid. The
residual gas, carbon dioxide, is passed through potassium hydroxide. Carbon
dioxide is a slightly acidic gas thus will react with the potassium hydroxide.
So basically the question is asking how much carbon dioxide is produced since
all of it reacts with the potassium hydroxide. 1 mole of methane has 1 carbon
atom thus will produce 1 mole of carbon dioxide.1 mole of ethane has 2 carbon
atoms thus will produce 2 moles of carbon dioxide. A total of 3 moles of carbon
dioxide will be formed. In this case since all volumes are taken at same
temperature and pressure we can take the initial volumes of methane and ethane
as the number of moles of each present. 10 cm3 of methane gives
10 moles of methane. 10 cm3 of methane gives 10 moles of
ethane. Total of 30 moles of carbon dioxide are formed which corresponds to 30
cm3 of carbon dioxide.
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