While Ka measures the strength of an acid compound, the strength of an aqueous acid solution is measured by pH, which is an indication of the concentration of hydronium in the solution.
wˌaɪl kˈɑː mˈɛʒɚz ðə stɹˈɛŋθ əvən ˈæsɪd kˈɑːmpaʊnd, ðə stɹˈɛŋθ əvən ˈeɪkwɪəs ˈæsɪd səlˈuːʃən ɪz mˈɛʒɚd baɪ pˈiː ˈeɪtʃ, wˌɪtʃ ɪz ɐn ˌɪndɪkˈeɪʃən ʌvðə kˌɑːnsəntɹˈeɪʃən ʌv haɪdɹˈoʊniəm ɪnðə səlˈuːʃən.