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If we know the formula of the compound we can look at the ratio of the ions in the compound to determine the number of moles of potassium ions. First, we will look at potassium chloride the charge on potassium in an ionic compound is +1 and -1 for a chloride ion. As a result, we need one K+ ion and one Cl- ion to form a neutral compound resulting in a formula of KCl. if there is 0.1 mol of chloride, then we must have 0.1 mol of potassium because the formula has a 1:1 ratio between the ions. Now, we need to look at potassium carbonate. We already stated that potassium has a +1 charge. Carbonate has a -2 charge so we need two potassium ions to pair with one carbonate ion to form a neutral compound. The formula will be K2CO3 resulting in a ratio of 2:1 potassium ions to carbonate ions. Since we have 0.1 mol of carbonate, we need twice as many moles of potassium ions. So we will have 0.2 moles of potassium needed for the carbonate. Therefore the total moles of potassium will be 0.1 + 0.2 = 0.3 moles of potassium.
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