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Which is the strongest acid here?

  • $\ce{HClO}$
  • $\ce{HClO2}$
  • $\ce{HClO3}$
  • $\ce{HClO4}$

I found this question, didn't really get the first answer, but this one in particular seemed interesting.

Apparently, you calculate a $\mathrm{p}K_\mathrm{a}$ for each substance.

According to this website.

$$\mathrm{p}K_\mathrm{a} = -\log_{10}K_\mathrm{a}$$

I need to find out $K_\mathrm{a}$ then. From what I understand $K_\mathrm{a}$ is obtained from dividing the concentrations of products by the concentrations of reagents. However, how am I supposed to do so if I am not given any equilibrium reactions?

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In such questions you're supposed to know certain rules of acidity. You can find some of them on this page. In your case fourth rule, concerning oxoacids, is relevant and as you can see there, it's described on this very case. More precise formulation is so called Pauling's Rule described here. In general the more oxo groups connected to central atom, the more acidic compound.

Related: How are Pauling's rules applied to Nitric acid?

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