In the following tutorial, we often abbreviate $ H_3O^+ $ by $ H^+ $ (Which is actually false, indeed, $ H^+ $ is always fully hydrated in water!)

Which of the following mixtures is (are) a buffer(s) ?

a) $NH_3$and $NH_4Cl$ b) $HCl$ and $NaCl$ c) $H_2O$ and $NaOH$ d) $HCOOH$ and $HCOONa$

For answers, use (possibly several times) the arrows ↑ Down! and ↓ Up! Complete please this question before moving on to the next one!

Here is the correct answer: a) and d) B) is not a buffer because it is a strong acid $ HCl $ with its conjugate base $Cl^-$ C) is not a buffer because $ OH^- $ can be considered as a strong base with its conjugated acid $H_2O$