pH of Aqueous Salt Solutions

If you titrate acetic acid with sodium hydroxide, what is the pH at the equivalence point?

If you dissolve ammonium chloride in water, what is the pH of the solution?

To answer these questions you will have to calculate the pH of an aqueous salt solution!

AUS-e-TUTE has just added new resources to help you do just that!

Members should log-in to use the new tutorial, game, test, drill, worksheet wizard, and problem solving template.

If you are not an AUS-e-TUTE Member, you can access the "free-to-view" tutorial at https://www.ausetute.com.au/phsaltcalcs.html

Question Impossible?

This was the first question on the 2019 Western Australia ATAR Chemistry Course Examination 2019

See if you can answer it ...

Boric acid, which is a weak acid, was titrated with standardised sodium hydroxide
solution.

Which one of the indicators listed below would be the most suitable to use in this
titration?

	Indicator	Range of colour change  (pH)
(a)	thymol blue	1 – 3
(b)	bromocresol green	3.8 – 5.4
(c)	cresolphthalein	8 – 10
(d)	alizarin yellow	10 – 12

(Note: no further information was given in the question nor in the accompanying Data Book).

You've probably narrowed the answer to either (c) or (d) using a couple of common assumptions:

• these are aqueous solutions of acid and base

• the temperature of the solutions is 25C

The salt of a weak acid and a strong base in aqueous solution at 25C will have a pH greater than 7.

So, what now? We need more information, like the value of the acid dissociation constant for boric acid and some information about concentrations and volumes.

Boric acid is a weak acid, Ka = 5.8exp-10 (this information was not provided!)

Because Ka is so low, boric acid is essentially a monoprotic acid, so the salt produced is essentially NaH2BO3

Sodium ions will not hydrolyse but H2BO3- will hydrolyse.

H2BO3- + H2O --> H3BO3 + OH-

Kb for the hydrolysis of H2BO3-: 
Kb = Kw/Ka 
= 1exp-14/5.8exp-10 
= 1.7exp-5
We need to know the concentration of the salt,
 this wasn't given in the question,
 so we are going to assume a 1.0 mol/L solution 
(because the numbers are nice)
[H3BO3] = x
[OH-] = x
[H2BO3-] = 1.0 - x 
and assume x is negligible compared to 1.0
 therefore [H2BO3-] ~ 1.0

Kb = [H3BO3][OH-]/[H2BO3-]

1.7exp-5 = x2/[1.0]
take the square root of both sides:
x = 4.1exp-3 = [OH-]
pOH = -log10 [OH-] 
= -log[4.1exp-3] 
= 2.4

For aqueous solutions at 25C:
pH = 14 - pOH 
= 14 - 2.4 
= 11.6

So..... drum roll please .... the answer is (d)


Ofcourse, if the concentration of salt was 1exp-5 mol/L, 
the pH of the solution would be 9.1 .... 
and then the answer would be (c)

If you were unfortunate enough to sit this exam...
 then the answer according to the examiners report was (c) ...
but no details of how they arrived at this answer are provided.

Bleach

Lurking in my laundry cupboard is an opaque, plastic bottle containing bleach. I use it to whiten white clothes when they start to look a bit grey, but I also use it to clean blood stains, to remove mildew from walls and ceilings, and to remove the mould from bathroom tiles. The active ingredient in laundry bleach is sodium hypochlorite, NaOCl, and you will find a similar substance present in the "liquid chlorine" you add to swimming pools as a disinfectant.

Sodium hypochlorite is a very useful chemical, but it can also be dangerous if used incorrectly. 

The September 2018 issue of AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter for chemistry students and teachers, looks at the chemistry of bleach.

If you would like to subscribe to AUS-e-NEWS please go to https://www.ausetute.com.au/ausenews.html

Writing the Formula of an Inorganic Salt

Organic chemistry deals with compounds of carbon.
Inorganic chemistry deals with compounds of any other element.
One of the earliest known classes of inorganic compounds were salts.
Salts are binary, ionic compounds, that is, salts are made up of two ions:

• a positively charged ion called a cation
• a negatively charged ion called an anion 

The International Union of Pure and Applied Chemistry (IUPAC) is establishing rules to help us name, and write the formula, for compounds.
In an earlier tutorial we looked at how we name inorganic salts, in this tutorial we look at how to write the formula of an inorganic salt.

Naming Salts

How do you name a salt?
There are several different ways to name salts (binary inorganic ionic compounds).
The most common method used in school as an introduction to naming inorganic compounds is based on compositional nomenclature.
AUS-e-TUTE has a step-by-step guide to how to name a salt if you know the formula of the salt:
https://www.ausetute.com.au/namiform.html

Salt from Seawater

About 200 Mt of sodium chloride are consumed every year in the world.

Some sodium chloride will be added to food, some will be used as a highway de-icing agent in cold climates, but most of it will be used in industries such as leather tanning, dye manufacturing, paper production, and in the production of other chemicals such as sodium carbonate, sodium hydroxide and chlorine.

The largest source of sodium chloride is the world's oceans.

So, how hard can it be to produce sodium chloride?

Surely you just evaporate the water off seawater and "hey presto", sodium chloride!

In reality, it's not quite that simple .....

Learn more in the September 2016 edition of AUS-e-NEWS.

To subscribe to AUS-e-NEWS, AUS-e-TUTE's free quarterly newsletter, email :

typing "subscribe" as the subject.
 

Hidden Salt in Food

Researchers at VicHealth and Deakin University compared how much salt people thought they consumed with how much salt they really had consumed and found that Australians were not only eating too much salt, but were also eating more salt than they thought they were!

Australians were found to be consuming between 8 and 10 grams of salt per day, about twice the amount recommended by the World Health Organisation (WHO) that recommends adults should eat less than 5 g of salt (a bit less than a teaspoon) per day. The "salt" they are referring to is "table salt" which has the chemical name "sodium chloride" and the chemical formula NaCl. Sodium chloride is actually an ionic substance made up of sodium ions (Na⁺) and chloride ions (Cl^-) in a ratio of 1:1 and it is the sodium ions (Na⁺) that are the cause for concern because elevated levels of sodium ions increase a person's risk of high blood pressure, heart disease and stroke. Unfortunately, non-Chemists often refer to this as elevated "sodium" levels rather than as elevated "sodium ion" levels.

If you want to reduce your sodium chloride intake, the first thing you can do is NOT add "table salt" to your food when you eat it. However, only about 20% w/w of our daily intake of sodium ions comes from adding sodium chloride to our food at the table before we eat it. The other 80%  w/w of the sodium ions we consume is already present in our food, either naturally or because it has been added during processing.

Natural sources of sodium ions in our food include:

• milk and cream: 50 mg of sodium ions per 100 g
• eggs: 80 mg of sodium ions per 100 g
• carrot: 69 mg of sodium ions per 100 g
• spinach: 79 mg of sodium ions per 100 g
• green beans, potatoes: 6 mg of sodium ions per 100 g
• pumpkin: 1 mg sodium ions per 100 g
• apple, banana, pear: 1 mg sodium ions per 100 g

By far the greatest source of sodium ions in our diet comes from eating processed foods:

• 1 slice of white bread (30 g) can contain 140 mg Na⁺
• 1 slice of cheddar cheese (20 g) can contain 140 mg Na⁺
• 1 foil pack of butter (7 g) can have 55 mg of Na⁺
• 1 small bowl of breakfast cereal (30 g) can have 140 mg of Na⁺
• 1 small packet of potato chips (45 g) can have about 300 mg of Na⁺
• 1 can (12 fl oz, about 350 mL) diet coke has 40 mg of Na⁺
• tap water contains about 20 mg Na⁺ per 1 L

Food you buy in packets from a supermarket will have a list of ingredients and you can read this to find the amount of sodium ions (Na⁺) present in the food.

However, there are many foods we buy that do not come in a packet which tells us how much sodium ion is present. These foods make up our "hidden salt intake". You may find this information on company websites, such as

• 1 McDonalds Big Mac contains 859 mg of Na⁺
• 6 KFC chicken nuggets with sauce has 1040 mg of Na⁺
• 1 slice (¹/₈ ^th) of medium pan Pizza Hut Meat Lover's pizza has 740 mg of Na⁺
• 1 Taco Bell black bean burrito contains 1030 mg of Na⁺

Many people forget that sodium ions are also present in many medicines. Effervescent medicines contain sodium hydrogen carbonate (or sodium bicarbonate) which helps them dissolve in water.
For example,:

• 1 effervescent Berocca tablet contains about 280 mg of Na⁺ while the film-coated Berocca tablet contains only 1.85 mg of Na⁺
• 1 Gaviscon Advance tablet contains 55 mg Na⁺ but 10 mL of liquid Gaviscon contains 141 mg of Na⁺
• 1 Panadol Actifast caplet contains 173 mg Na⁺ but 1 Panadol soluble tablet contains 428 mg of Na⁺

So, if you want a diet that's low in sodium, eat lots of fresh fruit and vegetables, drink lots of water, and avoid packaged food and "fast food", and, remember to choose "low sodium" medicines.

Reference:
http://www.abc.net.au/news/2016-03-24/reducing-salt-intake-could-save-thousands-of-lives-each-year/7274140

Further Reading:
Mass Conversions
Percent by Mass

Suggested Study Questions:

1. Convert the following masses in grams to masses in milligrams:
   • 5 g
   • 8 g
   • 10 g
   • 30 g
   • 100 g
2. Convert the following masses in milligrams to masses in grams:
• 1.85 mg
• 55 mg
• 69 mg
• 173 mg
• 859 mg
• 1040 mg
3. Calculate the percentage of sodium ions and the percentage of chloride ions in sodium chloride.
4. Use the information in the article to calculate the mass of sodium ions each adult Australian currently consumes as a result of :
   • adding table salt to food before eating it
   • table salt that is naturally present or is added to food during preparation
5. 100 g of milk contains 50 mg of Na⁺ . What is the percentage by mass of sodium in the milk?
6. One 30 g slice  of white bread contains 140 mg Na⁺. What is the percentage by mass of sodium in white bread?
7. For lunch, a student ate a sandwich made up of 2 slices of white bread, 2 foil packs of butter and a slice of cheddar cheese. She also ate a 200 g banana, and washed it all down with 250 mL of plain, unflavoured milk.
   • Calculate the mass of sodium ions the student consumed for lunch.
   • Calculate her consumption of sodium ions as a percentage of the WHO recommended daily intake of sodium ions.
8. A different student consumed a Big Mac, 1 can of diet coke, and a 200 g packet of potato chips.
• Calculate the mass of sodium ions the student consumed for lunch.
• Calculate her consumption of sodium ions as a percentage of the WHO recommended daily intake of sodium ions.
9. For her birthday, a Chemistry Teacher's class gave her a 500 g block of dairy milk chocolate. The label included the information that the block of chocolate contained 82 mg of sodium per 100 g.
   • What is the mass of sodium ions in the block of chocolate?
   • If each person in the class of 22 received an equal share of the block chocolate, what mass of sodium ions would each person consume?
10. In Australia, the maximum recommended dose of paracetamol (the active ingredient in panadol tablets) is 4000 mg per day. 1 soluble Panadol tablet contains 500 mg of paracetamol.
    • What is the maximum number of soluble Panadol tablets per day that an adult Australian should consume?
    • If an adult Australian consumed the maximum recommended dose of soluble panadol tablets in 1 day, what mass of sodium ions would they have consumed?
    • What percentage of the WHO recommended maximum intake of sodium would this amount of panadol be?