pH After Mixing Weak Acids

 Acetic acid is a weak acid (K_a = 1.8×10^-5).  The pH of 100 mL of 0.10 mol L^-1 acetic acid is 2.9.

Hydrocyanic acid is a weak acid (K_a=6.3×10^-10). The pH of 100 mL of 0.10 mol L^-1 hydrocyanic acid is 5.1.

But what is the pH of the resultant solution if you mixed these two weak acids together?

AUS-e-TUTE has just added new resources to help you understand how to calculate the pH of solutions after mixing weak acids together. AUS-e-TUTE Members should log-in to access the new tutorial, game and test with worked solutions.

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

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

Monoprotic acid pH with a concentration of 0.022M

Question: What is the pH of a monoprotic acid with a concentration of 0.022 M?

Answer:
1. Assume this is a strong monoprotic acid so that it fully dissociates: HA → H⁺ + A^-

2. [HA] = [H⁺] = 0.022 M

3. pH = -log₁₀[H⁺] = -log₁₀[0.022] = 1.7

Find out more, and see questions with worked solutions, at https://www.ausetute.com.au/phstronga.html

What is the weight/volume percent (w/v) of a solution prepared from 50 g NaCl and 2.5 L of water?

Question: What is the concentration, weight/volume percent (w/v), of a solution prepared from 50. g NaCl and 2.5 L of water?

Answer:
1. w/v % is a measure of the concentration of a solution.
Units are often given as g solute/100 mL solvent.

2. w/v% = (mass solute in grams/volume of solution in mL) × 100

3. w= mass solute = 50 g
v = volume of solution = 2.5 L = 2.5 L × 1000 mL/L = 2500 mL (assuming the NaCl does NOT increase the volume as it dissolves in the water)

4. w/v% = (50 g/2500 mL) × 100 = 2 g/100 mL

Find out more, and see worked solutions to questions, at https://www.ausetute.com.au/wtvol.html

Gold Nuggets in 2019

Between 1850 and 1900, the city of Bendigo in Victoria, Australia, was the centre of a Gold Rush. Central Deborah Gold Mine, the last commercial gold mine to operate in Bendigo, re-opened as a tourist attraction in 1986.
Gold can still be found in the Bendigo area.
On Mothers' Day 2019, a family out walking Lucky their dog on the outskirts of Bendigo walked onto a gold nugget. They took it along to the local IGA supermarket to weigh it. The gold nugget weighed 624 grams (about 20 ounces).
Today, the price of gold in Australia is listed as $65.76 per gram. So, if pure, the gold in this nugget would be worth 624 × $65.76 ≈ $41,000.
How big would this gold nugget be?
We know the mass of the nugget is 624 grams.
We can look up the density of gold in tables, ρ = 19.3 g cm^-3
Since density = mass (g) ÷ volume (cm³)
19.3 = 624 ÷ volume (cm³)
volume (cm³) = 624 g ÷ 19.3 g cm^-3 = 32.3 cm³
Which could be represented by a cube approximately 3.2 cm × 3.2 cm × 3.2 cm
Not very big at all is it!

Ballarat, another Victorian Gold Rush town and site of the historic "Eureka Stockade", was also the place where another spectacular gold nugget was found in June 2019 (and reported nationally in July 2019). This gold nugget weighed about 2 kilograms, or 2,000 grams.
At today's prices, it would have a value of about 2,000 × $65.76 ≈ $130,000
And how big would this nugget be?
density = mass (g) ÷ volume (cm³) volume (cm³)
density = mass (g) ÷ density (g cm^-3) = 2,000 g ÷ 19.3 g cm^-3 = 103.6 cm³
The dimensions of a cube with this volume would be about 4.7 cm × 4.7 cm × 4.7 cm
Which would fit nicely into the palm of your hand as shown in the photograph below

Further Reading:
Density Calculations

Suggested Study Questions:

1.  The density of gold is 19.3 g cm^-3. Calculate the mass of
   • 1 cm³ of gold 
   • 10 cm³ of gold 
   • 1 m³ of gold
2.  The density of gold is 19.3 g cm^-3. Calculate the volume of
   • 1 g of gold
   • 10 g of gold
   • 1 kg of gold 
3.  The density of gold is 19.3 g cm^-3. Calculate the dimensions of a cube of gold which has a mass of
   • 5 g
   • 500 g
   • 5 kg
4. The density of gold is 19.3 g cm^-3. Calculate the diameter of a sphere of gold which has a mass of
   • 2 g
   • 200 g
   • 2 kg
5. A credit card has the approximate dimensions 65 mm × 55 mm × 1 mm. Calculate:
   • volume of the credit card in cm³
   • mass of a gold credit card
   • value of a gold credit card if the cost of gold is $65 per gram
   • Why aren't "gold" credit cards really made out of gold?
6. Gold is one of the few metals that is found in nature as the "native" element (that is, it is found as the element and not in compounds). Explain why gold can be found in nuggets.
7. Name some other metals that can also be found in their native state (that is, found as the element and not as compounds). Explain why these metals can be found in their native state. 
8. Name a metal that is not found on Earth in its native state, and explain why it is not found in nature as the uncombined element.

pK

You've probably heard of pH, maybe even pOH, but what is pK?
Does this sound like brand of chewing gum to you?
Or does that "p" mean something?
Time to find out, with AUS-e-TUTE's new pK resources!

https://www.ausetute.com.au/pk.html

pH after mixing acid and base

If you add enough strong base to a strong acid you can neutralise the solution and its pH will be 7.
But what if you don't add enough base to the acid.
Will the solution be neutral?
Will the pH of the solution be 7?
These are all very good questions which is why AUS-e-TUTE has just added a new tutorial, game, test and exam on this topic.
AUS-e-TUTE Members should log-in to use these resources.

If you are not an AUS-e-TUTE Member yet, you can view the tutorial for free at https://www.ausetute.com.au/phmixsab.html

Effect of Temperature on GIbbs Free Energy

Imagine a reaction in which you decompose liquid water to produce oxygen gas and hydrogen gas:

H₂O_(l) → H_2(g) + ½O_2(g)

The reaction is endothermic, ΔH > 0, and the change in entropy is also positive, ΔS > 0
At room temperature and pressure this reaction is not spontaneous ( ΔG > 0).
But could I heat, or cool, the reaction sufficiently to make the reaction spontaneous?
Can I turn a nonspontaneous reaction into a spontaneous reaction by changing the temperature?
Want to know?
Go to https://www.ausetute.com.au/gibbstemp.html

AUS-e-TUTE Members can access the tutorial, game, test and exam by logging-in to the Members Only area of the website.

Gibbs Free Energy Calculations

For a chemical system, either a chemical reaction or a physical change, at a constant temperature and pressure we define a function called the Gibbs Free Energy (G) so that we can determine whether the system will be spontaneous or non-spontaneous:

• spontaneous if ΔG < 0 (ΔG is negative)
• non-spontaneous if ΔG > 0 (ΔG is positive)

In this new tutorial we will calculate the change in Gibbs free energy of a reaction at constant temperature and pressure (ΔG) using:

• standard absolute entropy of each reactant and product (S°)
• standard enthalpy of formation of each reactant and product (ΔH_ƒ°)

For a chemical system under standard conditions, we can calculate the change in standard Gibbs free energy using the equation shown below:

ΔG° = ΔH° - TΔS°

AUS-e-TUTE members can access the new Gibbs free energy calculations tutorial, game, test and exam when they log-in (Go to Physical Chemistry Heading, then "Thermodynamics").

If you are not an AUS-e-TUTE member, there is a "free-to-view" Gibbs free energy calculations tutorial currently available for evaluation purposes at https://www.ausetute.com.au/gibbscalc.html

Calculating Entropy Changes

If you stick some clean reactive metal, like magnesium, into an aqueous solution of hydrochloric acid... Hey presto! Bubbles of hydrogen gas! This gas production is a dead giveaway that the entropy of the system has increased.
But what if all the reactants and products are soluble in water and the entire chemical process occurs in aqueous solution. Has the entropy of the system increased or decreased?
Thankfully, chemists have a tool they can use to make quantitative predictions about the changes in entropy of a chemical system (ΔS). This tool is called the Standard Absolute Entropy (S°), and we have just added a new tutorial, game, test and exam to help you understand what this is and how it can be used.
AUS-e-TUTE Members should log-in to access the new resources under the heading "Thermodynamics".
If you are not already an AUS-e-TUTE there is a "free-to-view" tutorial currently available at https://www.ausetute.com.au/entropychange.html

How Many Electrons in an Atom?

For an atom of an element:
number of electrons = number of protons = atomic number of element

AUS-e-TUTE has a new tutorial, game, test and drill to help you learn and apply this idea.
AUS-e-TUTE members should log-in to access these new resources.

If are not an AUS-e-TUTE Member, there is a "free-to-view" tutorial currently available at:
http://www.ausetute.com.au/noelectron.html

Fun Calculations

Practicing the skills of writing chemical formula and calculating relative molecular masses, molar masses and percentage compositions can be a bit, .. well, ... a bit ..... boring (sh, don't tell anyone I said that). But, you can use chemspider to find some interesting molecules for your students to practice their skills on.
ChemSpider is a free chemical structure database run by the Royal Society of Chemistry. It provides access to over 25 million chemical structures, properties and associated information.

Activity:
For each of the molecules below:

1. Use chemspider to find the structural formula.
2. Write the molecular formula..
3. Calculate the relative molecular mass..
4. Calculate the molar mass..
5. Calculate the percentage of carbon present..

Molecules with "interesting names" to practice on:

• olympicene
• ovalene
• performic acid
• traumatic acid
• arabitol
• putrescine
• cadaverine
• nonose 
• syringic acid
• warfarin
• orotic acid
• megaphone
•  moronic acid
• inflatene
• sexithiophene
• arsole
• anol

Further Reading
Calculating Relative Molecular Mass
Molar Mass
Percentage Composition