Question:easy
In potash alum, the ratio of \(\text{K}^+\) and \(\text{SO}_4^{2-}\) ions is
In potash alum, the ratio of \(\text{K}^+\) and \(\text{SO}_4^{2-}\) ions is
Show Hint
Always remember the empirical formula of potash alum can also be written in its simplified form as \(\text{KAl}(\text{SO}_4)_2 \cdot 12\text{H}_2\text{O}\). In this format, you can read the ratio directly from the subscripts: there is $1$ potassium ion for every $2$ sulfate ions, giving the answer $1:2$ instantly.
Updated On: Jun 21, 2026
- \(3:2\)
- \(1:2\)
- \(2:1\)
- \(2:3\)
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Recall what potash alum is.
Potash alum is a double salt, potassium aluminium sulfate dodecahydrate. In water it breaks fully into its simple ions, so we just read the ion counts from its formula.
Step 2: Write the formula.
Potash alum is \(K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O\), often written per potassium as \(KAl(SO_4)_2\cdot 12H_2O\).
Step 3: Count ions in the simplest unit.
Take \(KAl(SO_4)_2\cdot 12H_2O\). It contains one \(K^+\) and two \(SO_4^{2-}\).
Step 4: Form the ratio.
So the ratio of \(K^+\) to \(SO_4^{2-}\) is \(1:2\).
Step 5: Cross-check with the double formula.
In \(K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O\) there are \(2\) potassium ions and \(1+3 = 4\) sulfate ions, which is again \(2:4 = 1:2\).
Step 6: State the answer.
The ratio of \(K^+\) to \(SO_4^{2-}\) is \(1:2\), option 2.
\[ \boxed{1:2} \]
Potash alum is a double salt, potassium aluminium sulfate dodecahydrate. In water it breaks fully into its simple ions, so we just read the ion counts from its formula.
Step 2: Write the formula.
Potash alum is \(K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O\), often written per potassium as \(KAl(SO_4)_2\cdot 12H_2O\).
Step 3: Count ions in the simplest unit.
Take \(KAl(SO_4)_2\cdot 12H_2O\). It contains one \(K^+\) and two \(SO_4^{2-}\).
Step 4: Form the ratio.
So the ratio of \(K^+\) to \(SO_4^{2-}\) is \(1:2\).
Step 5: Cross-check with the double formula.
In \(K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O\) there are \(2\) potassium ions and \(1+3 = 4\) sulfate ions, which is again \(2:4 = 1:2\).
Step 6: State the answer.
The ratio of \(K^+\) to \(SO_4^{2-}\) is \(1:2\), option 2.
\[ \boxed{1:2} \]
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