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.