Moles and Reactions by Computer: Quick Revision

A brief note on Moles and Reactions by Computer. Read it before a test. Try each question at the end before you open its answer.

Formulas as data

• Keep atomic masses in a dictionary: atomic = {"H": 1.008, "C": 12.01, "O": 16.00}.
• Read a formula with re.findall(r"([A-Z][a-z]?)(\d*)", formula): one pair for each symbol, and an empty count means one.
• Open brackets first: the number after a bracket multiplies every atom inside.
• Molar mass: sum(atomic[el] * n for el, n in counts.items()). Mass percents add up to $100$.

Reactions as functions

• Keep an equation as two dictionaries of coefficients.
• Rounds $=$ moles $\div$ coefficient. min(rounds, key=rounds.get) gives the limiting reagent.
• Products: extent $\times$ coefficient; excess left: moles $-$ extent $\times$ coefficient.
• Percent yield $= \dfrac{\text{actual}}{\text{theoretical}} \times 100$; at STP a mole of gas fills $22.7\ \text{L}$.

Solutions

• Moles $=$ molarity $\times \dfrac{V \text{ in mL}}{1000}$.
• Dilution: $M_1V_1 = M_2V_2$. Mixing: add the moles and the volumes.
• End point: moles of base $=$ moles of acid $\times$ the ratio of the equation; a default argument ratio=1 keeps the common case short.

A worked example

What does this print?

1. Rounds. Phosphorus $\dfrac{0.10}{1} = 0.10$; oxygen $\dfrac{0.60}{5} = 0.12$.
2. Limiting. Phosphorus has fewer rounds, so min returns 'P4'.
3. Product. In $\text{P}_4 + 5\text{O}_2 \rightarrow \text{P}_4\text{O}_{10}$ the product's coefficient is $1$, so $0.1$ mol of $\text{P}_4\text{O}_{10}$ forms.

It prints P4 0.1.

Practice with new numbers

A program to try

The program reads a formula, finds its molar mass and the molarity of a solution made from it.

Mistakes to avoid

• A missing element. KeyError: 'K' means the dictionary has no potassium.
• int(''). An empty count raises ValueError; write int(n or 1).
• Comparing moles instead of rounds. Divide by the coefficient first.
• Millilitres as litres. Divide by $1000$.
• Adding molarities when mixing. Add the moles, then divide by the total volume.

Check yourself

1. How many atoms of each element does a correct reader count in Zn(NO3)2?

2. For $\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$ with $1$ mol of each gas, which gas does the function name as limiting?

3. A function returns 6000.0 mol for $0.3\ \text{mol/L}$ in $20\ \text{L}$. What is wrong, and what is the right answer?

4. Why should you test a molar mass function on a formula with brackets?