Science Makes Sense, Week 13: Strong Acids, Bases, logarithms, pH, moles, Avogadro’s number.

During my undergraduate days of studying chemistry, I remember a friend of mine who was in love with the subject. She would say,” I want to keep my hands dipping in acid all my life!” She could never have done that literally because most acids are corrosive liquids. It was her unique and maybe bizarre way of bonding with the subject.

All acids have a sour taste and definitely are corrosive. We shall focus on some common inorganic acids; hydrochloric acid, (HCl), nitric acid (HNO₃) and sulfuric acid (H₂SO₄). As you can see, the acids all have hydrogen ions H⁺ (cations). The acids are ionic and exist as hydrogen ions and chloride/nitrate/sulfate ions (anions) as shown below:

HCl → H⁺ Cl^–

HNO_3 →H⁺NO₃^–

H₂SO_4 →2H⁺ SO₄^2-

(Note that the sulfate ion has two negative charges, so with the 2 hydrogen ions it will be neutral.)

The more complete this dissociation is the stronger the acid is; in other words the concentration of hydrogen ions is very high here. These 3 acids are considered strong acids along with the following: hydrobromic acid (HBr), hydroiodic acid (HI) and perchloric acid (HClO₄).(Ref. 1)

The negative logarithm of the number of hydrogen ions is called the pH of the acid. Imagine you have a 0.01M solution of HCl. 1 mole=1M means there is 1 atomic mass in grams/g for H and Cl which is the formula for HCl, which means there is 1+ 35.5=36.5g of HCl in 1liter of distilled water. 0.01M solution will be made taking 1 ml of this solution and adding 99ml of distilled water; one-hundredth of 1M.

The pH of this solution will be -(log0.01)= -(log 10^-2 )=-(-2)=2. Strong acids usually have a low pH near 2 or below that value.

Common acids in the house include toilet bowl cleaners (which contain some hydrochloric acid, HCl); industrial uses of acids include making fertilizers, dyes, in photography and explosives manufacturing to name a few.(Ref.2,3)

Bases are usually soapy to touch and some are very corrosive. All inorganic bases have a cation and an anion like acids, but the cation is not a hydrogen ion but usually a metal ion like lithium, sodium, potassium, etc. The anion is always a hydroxyl ion which is an OH negative ion shown like this:OH^–

Common bases are ionic like acids and are written as sodium hydroxide, NaOH, potassium hydroxide,KOH, calcium hydroxide,Ca(OH)₂ etc but exist actually as shown for acids above, viz.,

NaOH → Na⁺OH^–

KOH → K⁺OH^–

Ca(OH) → Ca⁺⁺2OH^–

Again, the greater the dissociation of hydroxyl ions, the stronger is the base. So bases are characterized by the number of hydroxyl ions, OH^– and end with hydroxide. You can calculate the pOH of the base just like for the acids. Again, imagine you have a .01M solution of sodium hydroxide, Na⁺OH^–and so the pOH of the base will be:

-(log 0.01) = -(-2)= 2. Now pH + pOH =14, hence pH = 14-2=12. pH of acids is below 7 and pH of bases is between 7 and 14. Strong bases will have more hydroxyl ions and will be around 11 or 12 or even higher. Water is neither a base nor an acid and is neutral, so the pH will be around 7.

Bases like sodium hydroxide is used for making soap. Potassium hydroxide is used in alkaline batteries and calcium hydroxide is used in bleaching powder and as an antidote for food poisoning.(Ref.4)

Next week, we will look at the reactions between acids and bases and study the resulting products.

Activities for Middle School Teachers:

Compare words like’volume’ , ‘mole’,’rational’ and ‘irrational’, generally used in the English language and in Science and Mathematics. Let students find out other words that have different meanings inside and outside the field of Science/Mathematics, like ‘litmus test’.

pH uses logs. Study logs or what is called logarithms. Logs are to the base 10 ; hence log 10 is actually 1. This can also be stated as the exponent of 10 is 1 when the base is 10 or 10¹ =10. Log 100 =2, because the exponent of 10 has to be 2 to equal 100. (Before calculators were invented, log tables were used to do complicated multiplication, division, and exponential calculations.)

Using different molarities for hydrogen ion concentrations in acids and hydroxyl ions in bases, calculate the pH of different acids and bases.

Find out more about Avogadro and Avogadro’s Number.

Why do gardeners worry about the pH of the soil? What plants need acidic soil and why?

Nuggets of Information:

Hydrochloric acid,HCl, mixed with nitric acid.HNO_3,is called aqua regia, literally meaning ‘royal water’. This mixture is used in metallurgy to dissolve metals like gold Au, and platinum, Pt.(Ref. 5)

HCl is also known as muriatic acid and used to clean concrete. (Ref.2)

Did you know that when you usually eat on time and then sometimes you don’t, your stomach growls? The growling is actually the secretion of hydrochloric acid, HCl, inside the walls of your abdomen getting ready to digest the food that should be there! (Ref.2)

Battery Acid in cars is sulfuric acid, H₂SO₄.(Ref. 6)

What is a mole? Normally a mole means an animal that lives underground. But in chemistry, a mole is a number like a dozen(12) or a gross(144). A mole is really a unit of measurement; it is a number that has the same number of particles found in 12.0 gms of Carbon-12= Avagadro’s number. This number is 6.023x 10²³ . 1 mole of carbon-12 has 6.023x 10²³ atoms. 1 mole of teachers = 6.023x 10²³(Ref.7) , 1mole of any element will be its atomic mass and will have 6.023x 10²³atoms.

Calcium Hydroxide is also known as slaked lime, sodium hydroxide is called caustic soda or lye. The latter is used in petroleum refining, in medicines and in the manufacture of rayon. (Ref.4)

A base is also called an alkali. The word ‘alkali’ is derived from Arabic, since the ancient Muslims in the Middle East were well-known chemists. Al Qali means the ashes.(Ref. 8)

Another well-known base/alkali is ammonium hydroxide written as NH₄OH , but exists in the ionic form NH₄⁺OH^– like all the other bases and acids. Dawn, a dish washing liquid, contains ammonium hydroxide, or ammonia as it is commonly called. Ammonium Hydroxide is used in the cosmetic industry, and also as a spot and grease remover.(Ref.4)

Ammonia and water is in equilibrium with ammonium hydroxide :

NH₃+ H₂O → NH₄OH

‘Smelling salts’ have existed since the time of the Romans and was used by women whenever they had a fainting fit. It contains water and ammonia which is essentially ammonium hydroxide (above equation). Even today, athletes in football and hockey tend to use ‘smelling salts’ to give them some boosts of energy. The fear is the abuse of this base.(Ref. 9)

A few years ago there was a major controversy with pink slime, the addition of ammonium hydroxide in meat. McDonald’s insisted that they had stopped its use in their products.(Ref. 10)

Don’t forget, sodium hydroxide is used to unclog drains and is present in the famous household product, Drano.(Ref.11)

Indicator liquids change color in acidic or basic solutions; most are organic weak acids themselves. Phenolphthalein turns bright pink in basic solutions and is colorless (original color) in acidic solutions. Red litmus paper turns blue in basic solutions and blue litmus paper turns red in acidic solutions. There is also a universal indicator paper that changes color based on the pH of the solution.(Ref. 12 )

Acids also have the ability to react with metals like zinc, magnesium to produce hydrogen gas. (Ref. 13)

References:

Tags: acids, Avogadro's Number, bases, logs, moles, pH

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