Page 22 - Focus SPM KSSM F4 2020 - Chemistry

Chemistry Form 4 Chapter 3 The Mole Concept, Chemical Formula and Equation
Ionic compound: Iron(III) chloride
① Based on its name, write the formula of
the cation followed by the formula of
the anion. Fe 3+ CI -

② Determine the number of cations and 1 Fe ion 3 CI ions
-
3+
anions by balancing the positive Total positive charges Total negative charges
charges and negative charges.
= 1(+3) = 3(-1)
= +3 = -3
③ Write the number of cations and anions
as subscript numbers. Formula : FeCI
3
Figure 3.12 Steps in constructing the formula of an ionic compound

SPM Tips
EXAMPLE 3.23
Chapter
3 Write the formula of sodium sulphate. Students need to memorise the formulae of cations
and anions. If not, the chemical formulae cannot be
Solution constructed correctly.
Ionic compound: Sodium sulphate
3. When polyatomic ions such as OH , NO ,
–
–
3
+
2-
Na + SO 2– SO and NH are involved, brackets are used
4 4 4
to show the number of ions in the formula. See
Figure 3.12.
+
2 Na ions: 1 SO 4 2– ions:
Total positive charges Total negative charges Mg(OH) Fe(NO ) (NH ) SO
= 2(+1) = 1(-2) 2 3 3 4 2 4
= +2 = -2 The brackets The brackets The brackets
show there are show there are show there are
2 groups of 3 groups of 2 groups of
Formula : Na SO OH – NO – NH +
2 4 3 4
Figure 3.13 Brackets are used to show the number
of polyatomic ions
SPM Highlights
The formula of calcium chloride and potassium bromate(V) are CaCl and KBrO . What is the formula of calcium
3
2
bromate((V)?
Examiner’s tip
From the formula of KBrO , 1 ion K is needed to balance 1 bromate((V) ion. So, it can be deduced that the formula
+
3
of bromate(V) ion is BrO . -
3 Ionic compound: Calcium bromate(V)

Ca 2+ BrO 3 –

1 ion Ca : 2 ion BrO :
–
2+
3
Total positive charges Total negative charges
= 1(+2) = 2(-1)
= +2 = -2
Formula : Ca(BrO )
3 2

50

03 SPM CHEMISTRY F4.indd 50 27/02/2020 11:23 AM