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VICChemistryQuick questions
Unit 1: How can the diversity of materials be explained?
Quick questions on Empirical and molecular formulae (VCE Chemistry Unit 1)
9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is empirical formula?Show answer
The simplest whole-number ratio of atoms in a compound. CH$_2$O is the empirical formula for glucose, ethanal, methanal and many other compounds.
What is molecular formula?Show answer
The actual number of each atom in a molecule. Glucose is C$_6$H$_{12}$O$_6$, a multiple of the empirical CH$_2$O.
What is step 1?Show answer
Convert CO$_2$ mass to moles, then to mass of C ($\times 12.01$ from $44.01$).
What is step 2?Show answer
Convert H$_2$O mass to moles, then to mass of H ($\times 2 \times 1.008 / 18.02$).
What is step 3?Show answer
Subtract C and H masses from original sample mass; if any mass remains, that is mass of O (assuming only C, H, O in the compound).
What is step 4?Show answer
Proceed as for percent composition.
What is forgetting to divide by smallest?Show answer
Reasoning by inspection often fails on three- or four-element compounds.
What is rounding too early?Show answer
Keep $3$-$4$ significant figures until the final step; rounding $1.5$ to $2$ early hides an empirical formula that should have been multiplied by $2$.
What is confusing empirical with molecular when molar mass is given?Show answer
Always compute the multiplier.