A set of chemical calculations involving the mole and chemical equations are given on 12 Feb to 5PR during Structured Revision Programme.
Please study the steps taken to solve each of the 4 problems from the section on Calculations From Equations (p4 to 6)
Q1(a) No. of mole of iron used = mass of iron used / atomic mass of iron = 28/56 = 0.5 mol
From the balanced equation, 1 mole of iron produces 1 mole of iron(II) chloride.
Hence, 0.5 mol of iron used produces 0.5 mol of iron(II) chloride.
To find mass of iron(II) chloride formed,
Molar mass of iron (II) chloride = 56 + (35.5 x 2) = 127g. (this means mass of 1 mole of iron(II) chloride is 127g)
Mass of iron(II) chloride found in 0.5 mol = 0.5 x 127g = 63.5g
Q1(b) Use Avogadro's Law (1 mole of any gas occupies a volume of 24 dm3 at room temperature and pressure (r.t.p.)) to solve this problem.
From the equation, 1 mole of iron produces 1 mole of hydrogen gas.
Therefore, 0.5 mol of iron (from 1(a)) produces 0.5 mol of hydrogen at r.t.p.
Volume of hydrogen produced = 0.5 x 24 = 12 dm3.
Q2. Step 1: No. of moles of CuCO3 heated = 74.4 / (molar mass of CuCO3)
= 74.4 / (64 + 12 + 48)
= 0.6 mol
Step 2: State that 1 mole of CuCO3 decomposes on heating to produce 1 mole of CuO.
Hence, 0.6 mol of CuO is produced from heating 0.6 mol of CuCO3 (from step 1).
Step 3: Therefore, mass of CuO (in 0.6 mol) = 0.6 x (molar mass of CuO)
= 0.6 x (64 + 16)
= 48 g
Q3. Same problem solving method as Q1b.
No. of moles of HCl reacted (14.8 g) = 14.8 / (1 + 35.5) = 0.406 mol.
From the equation, 2 moles of HCl reacts to produce 1 mole of sulphur dioxide (SO2) gas.
Hence, if 0.406 mol of HCl has reacted, 0.406 / 2 = 0.203 mol of SO2 gas is produced.
Using Avogadro's Law, volume of SO2 produced at r.t.p. = 0.203 x 24 dm3
= 4.87 dm3 (3 s.f.)
Q4. Same problem solving method as Q2.
Work out number of mole of copper in the 10.8g used (0.169 mol)
Find number of mole of aluminium produced from the chemical equation (0.113 mol)
Work out mass of aluminium solid in 0.113 mol (3.04 g) [Atomic mass of Al is 27].
Key point - You must convert all mass (in g or kg) and volume (in cm3 or dm3) into the equivalent values in mole (mol) to use the chemical equation to compare and work out respective amounts.
Wednesday, February 18, 2009
5 Praise Sci(Chem) - Comments on Acids & Bases Assignment (Mr Kom)
Where? Textbook Q2(p237), Q2(p239), Q1,2,5 (pp241,242)
Through marking your written assignments, I have some important observations to bring to your attention.
On Q2a (p237)
1. Incorrect formula for calcium nitrate. Should be Ca(NO3)2, not CaNO3. Reason is that calcium atom can only lose 2 electrons to form Ca2+ ion. Hence, you'll need two NO3- (nitrate) ions.
2. The question specifically asked to show the chemical formula of the salt formed, not to write the chemical equations which some of you did.
On Q2b, c and d (p237)
1. This question focuses on checking whether you know which of the five methods of salt preparation (four for preparing soluble salts, one (precipitation) for insoluble salt) can be used to prepare a particular salt.
First, you need to know if the salt is soluble or insoluble in water (for this, you simply need to refer to (and remember) Table 15.4 on p233). Then use the diagram (figure 15.10 on p233) to help you choose the most suitable salt preparation method. The key is salt solubility.
The more detailed process of each of the five salt preparation methods are found from p234 to p236).
One common error for 2d is that many of you did not write the state symbols in the equations.
On Q1 (p241)
1c. Please show state symbols. (Potassium chloride (KCl), a soluble salt, can be prepared during the titration method).
1d. Some of you failed to write down the chemical name of the salt formed.
1e. The ionic equation: H+ (aq) + OH- (aq) --> H2O (l) is the standard equation for neutralization reactions. Below shows how this equation is derived:
Chemical Equation: KOH (aq) + HCl (aq) --> KCl (aq) + H2O (l)
K+ + OH- + H+ + Cl- --> K+ + Cl- + H2O (l) (Ionize only those ionic
compounds with (aq) state symbols
Cancel spectator ions, K+ & Cl-. They do not take part in the reaction even though present in the solution.
On Q2 (p241)
a. Your explanation needs to bring out the idea of the word 'aqueous' which means 'dissolved in water'. Hence, a clearer explanation of aqueous ammonia is ' a solution formed when ammonia is dissolved in water'.
b. Many of you did not label your x-axis and y-axis.
If you sketch the graph without using graph paper, please sketch neatly and draw your graph line as accurately as possible.
c. (i) pH 4
(ii) The soil is weakly acidic. (Note that aqueous ammonia is a weak alkali)
(iii) pH10
(iv) 9 cm3 (must show unit). This corresponds to pH7 on the y-axis of the graph.
Through marking your written assignments, I have some important observations to bring to your attention.
On Q2a (p237)
1. Incorrect formula for calcium nitrate. Should be Ca(NO3)2, not CaNO3. Reason is that calcium atom can only lose 2 electrons to form Ca2+ ion. Hence, you'll need two NO3- (nitrate) ions.
2. The question specifically asked to show the chemical formula of the salt formed, not to write the chemical equations which some of you did.
On Q2b, c and d (p237)
1. This question focuses on checking whether you know which of the five methods of salt preparation (four for preparing soluble salts, one (precipitation) for insoluble salt) can be used to prepare a particular salt.
First, you need to know if the salt is soluble or insoluble in water (for this, you simply need to refer to (and remember) Table 15.4 on p233). Then use the diagram (figure 15.10 on p233) to help you choose the most suitable salt preparation method. The key is salt solubility.
The more detailed process of each of the five salt preparation methods are found from p234 to p236).
One common error for 2d is that many of you did not write the state symbols in the equations.
On Q1 (p241)
1c. Please show state symbols. (Potassium chloride (KCl), a soluble salt, can be prepared during the titration method).
1d. Some of you failed to write down the chemical name of the salt formed.
1e. The ionic equation: H+ (aq) + OH- (aq) --> H2O (l) is the standard equation for neutralization reactions. Below shows how this equation is derived:
Chemical Equation: KOH (aq) + HCl (aq) --> KCl (aq) + H2O (l)
K+ + OH- + H+ + Cl- --> K+ + Cl- + H2O (l) (Ionize only those ionic
compounds with (aq) state symbols
Cancel spectator ions, K+ & Cl-. They do not take part in the reaction even though present in the solution.
On Q2 (p241)
a. Your explanation needs to bring out the idea of the word 'aqueous' which means 'dissolved in water'. Hence, a clearer explanation of aqueous ammonia is ' a solution formed when ammonia is dissolved in water'.
b. Many of you did not label your x-axis and y-axis.
If you sketch the graph without using graph paper, please sketch neatly and draw your graph line as accurately as possible.
c. (i) pH 4
(ii) The soil is weakly acidic. (Note that aqueous ammonia is a weak alkali)
(iii) pH10
(iv) 9 cm3 (must show unit). This corresponds to pH7 on the y-axis of the graph.
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