A GUIDE TO ACIDS, ACID STRENGTH, AND CONCENTRATION What’s the difference between acid strength and concentration? And how does pH fit in with these? This graphic explains the basics.

CH COOH HCl H2SO4 HNO3 H3PO4 HF 3 H2CO3

HYDROCHLORIC ACID SULFURIC ACID NITRIC ACID PHOSPHORIC ACID HYDROFLUORIC ACID ETHANOIC ACID CARBONIC ACID

pKa = –7 pKa = –2 pKa = –2 pKa = 2.12 pKa = 3.45 pKa = 4.76 pKa = 6.37

STRONGER ACIDS WEAKER ACIDS

STRONG ACIDS VS. WEAK ACIDS ACIDS, Ka AND pKa CONCENTRATION AND pH

+ – The H+ ion is transferred to a + A decrease of one on the pH scale represents + [H+] [A–] pH = –log10[H ] a tenfold increase in H+ concentration. HA H + A water molecule, forming H3O Ka = pKa = –log10[Ka]

– [HA] – – + + A + + A– + A + A H + H H H H A H + H H H A Ka pK H – + – H a A H A A – + A– A + H A– H A– VERY STRONG ACID >0.1 <1 A– + H A + + + – H H A H A H H H + A – + – H A– A H A A– –3 FAIRLY STRONG ACID 10 –0.1 1–3 – – + A A + H – – + – H + H A A H A A A H H + A A– + H A– H H WEAK ACID 10–5–10–3 3–5 STRONG ACID WEAK ACID VERY WEAK ACID 10–15–10–5 5–15 CONCENTRATED ACID DILUTE ACID

+ – H Hydrogen ions A Negative ions H A Acid molecules EXTREMELY WEAK ACID <10–15 >15 H+ Hydrogen ions A– Negative ions

Acids react with water when they are added to it, The acid dissociation constant, Ka, is a measure of the Concentration is distinct from strength. It refers to the forming ions. The degree to which they do this is what strength of an acid. The higher its value, the stronger amount of acid in a given solution. A concentrated acid

determines whether they are strong or weak acids. the acid (i.e. the more readily it ionises in water). pKa contains a large amount of acid in a given volume; a

Strong acids are essentially 100% ionised in solution. converts Ka number to a logarithmic scale that makes it dilute solution contains a small amount. The pH scale Weak acids ionise very little in solution. easier to compare strengths of different acids. gauges the amount of hydrogen ions in solution.

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