Refer to aqueous solutions containing 1:1 mole ratios of the following pairs of substances. Assume all concentrations are 1 M.
Which solution has the lowest pH?

a) NH3 and NH4Cl
b) H3PO4 and NaH2PO4
c) HCl and NaCl
d) NaOH and NH3
e) NH2 and HC2H3O2 (acetic acid)

HCl and NaCl option c

Explanation:

 According to the pH scale interpretation a lower pH means that its is more acidic, while a higher pH means that it is more basic.

Simply put a pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

From the question above c fits in the option with the lower pH meaning its a strong acid and it tends more towards 0 than 7.

A solution, containing a buffer, is one that maintains a relatively constant pH.

NH3 and NH4Cl ;H3PO4 and NaH2PO4 are an acid/base conjugate pair each. They will make an excellent buffer.

NaOH and NH3 would rather make a slightly basic mixture

NH2 and HC2H3O2 (acetic acid) slightly basic mixture ..

Solution c has the lowest pH.