How can you tell whether a molecule is an acid or a base?
Does an OH make a base? Guess first, then look for the loose hydrogen and the lone pair, and work out why vinegar has a pH near 3.
- 1An acid gives away H+, a base takes it5 min
- 2Look for the loose hydrogen, and the lone pair7 min
- 3Strength is willingness, not amount6 min
- 4pH counts the free hydrogen ions, on a log scale6 min
Sound like you?
The ideas people most often get wrong on the way, in their own words. Each step starts from one of them.
The short answerRather read it than work it out?Show it (it gives the steps away)
Acid and base name jobs in a reaction. An acid donates a hydrogen ion, H+, and a base accepts one, so they always work as a pair. The same molecule can do either job: water takes H+ from HCl and gives H+ to ammonia. Asking 'acid or base?' means 'compared with what?'
To tell whether a molecule is an acid, look for a hydrogen on an electronegative atom whose loss leaves a stable ion, as in the O-H of acetic acid. To tell whether it is a base, look for a lone pair that can take a proton, as on nitrogen in ammonia. An OH in a formula is not hydroxide, and hydrogens on carbon essentially never leave.
A strong acid gives up essentially all its H+; a weak acid only a little. That is separate from concentration: a weak acid can be concentrated and a strong acid dilute. The pKa measures willingness: a lower pKa means a stronger acid, and each unit is a factor of ten.
pH is minus the base-10 log of the free H+ concentration. 0.1 mol per litre of HCl frees all of it, so pH 1. 0.1 mol per litre of acetic acid frees about 1.3%, [H+] = 0.00134, so pH 2.87. Each pH unit is a tenfold change, so the same amount of two acids can differ in acidity by tens of times.
Learn it with your own tutor.
Add it to your goals and Cognosc starts from what you already know, works through the rest with you, and keeps checking you still understand it.