O-Level Chemical Calculations: worked solution

Question

Complete this table of names and formulae: give the formula of zinc hydroxide; name the compound Na2SO4; name the compound FeCl3.

Zinc hydroxide - give the formula. Zinc always forms the 2+ ion, Zn2+, and hydroxide is OH-. Two OH- (2 x 1- = 2-) are needed to cancel the 2+ on one Zn2+. OH is a group of two atoms, so it must be put in brackets before the 2; writing ZnOH2 would wrongly mean one oxygen and two hydrogens. Formula: Zn(OH)2 Na2SO4 - give the name. Na is sodium, and the SO4 group carrying a 2- charge is sulfate. The 2 in front of Na is there because two Na+ (2 x 1+) are needed to balance one SO4²-. Sodium is in Group I and can only form the 1+ ion, so no Roman numeral is needed in the name. Name: sodium sulfate FeCl3 - give the name. There are 3 chloride ions, each Cl-, giving 3- in total. The compound is neutral overall, so the single iron ion must carry 3+, i.e. Fe3+. Iron can form more than one ion (2+ or 3+), so the charge has to be shown as a Roman numeral in the name. Name: iron(III) chloride