Quick questions on Chemical calculations - WJEC A-Level Chemistry

When two reactants are mixed in amounts that are not in the exact stoichiometric ratio, one runs out first and limits the amount of product. To find the limiting reagent, calculate the moles of each reactant, divide by its coefficient in the balanced equation, and the smallest value identifies the limiting reagent. All product calculations must then be based on the limiting reagent, not the one in excess. Forgetting to check which reactant limits the reaction is one of the most common errors in quantitative questions.

The equation $pV = nRT$ links the four state variables of a gas. The most frequent mistake is mixing units, so convert before substituting: pressure to pascals (Pa), volume to cubic metres (m$^3$, dividing cm$^3$ by $10^6$), and temperature to kelvin (adding $273$ to degrees Celsius). Rearranged as $n = pV/RT$, it lets you find moles of a gas from its measured pressure, volume and temperature, which can then be combined with a mass to find a molar mass, a standard way to identify an unknown gas.

Concentration and mole calculations underpin acid-base titrations used to check the purity of pharmaceuticals and the acidity of foods. Atom economy in industry. Manufacturers favour addition reactions (atom economy near 100 percent) over substitution because waste by-products raise cost and environmental burden, a recurring theme in Unit 2's wider-impact content.

Calculate the number of moles in $9.80$ g of sulfuric acid ($M = 98.0$ g mol$^{-1}$). [1 mark]

Calculate the concentration when $0.250$ mol of solute is dissolved in $500$ cm$^3$ of solution. [1 mark]

Calculate the atom economy for producing hydrogen from $\text{Zn} + \text{H}_2\text{SO}_4 \rightarrow \text{ZnSO}_4 + \text{H}_2$. [2 marks]

State how you identify the limiting reagent in a reaction. [1 mark]

State the units required for pressure, volume and temperature in $pV = nRT$. [1 mark]