Which of the following solutions will have a pH = 1.00?

Question

I first did the question and by using the -log to find pH I thought 1 and 2 would be correct, but the answer key says it is only 1. Can someone explain why please? Thanks!

Here are the answers you can choose from :

1. 0.1M HCL
2. 0.1M HNO2
3. 0.1M NaOH

Answer 1

HCl is a STRONG acid and ionizes 100% so the H⁺ concentration will be 0.1 M and pH = 1

HNO₂ is a weak acid and ionizes less than 100% so the H⁺ concentration will be less than 0.1 and pH will not be 1.