Question

How many grams of carbon dioxide can form when a mixture of 3.60 g ethylene (C2H4) and 4.10 g of oxygen is ignited, assuming complete combustion to form carbon dioxide and water?

Answer 1

Stoichiometry

When performing stoichiometric calculations the values to be used \(\sf \color{red}{must~be~in~moles}\). This is because the Molar Masses (\(\sf M_{m}\) of two elements are not the same. This is to say that 1 gram of Hydrogen atoms is not the same number of atoms as 1 gram of Nitrogen atoms. We need to be working with numbers of particles (e.g. atoms, molecules, formula units) and using moles is the way this is done.

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To interconvert between mass and moles, use the relationship:

\(\Large \sf moles=\dfrac{mass}{Molar~mass}\)

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General Scheme for Stoichiometric Calculations:

\(\sf \large 1.\)First write and balance an equation for the process described.
\(\sf \large 2.\)Next, use the stoichiometric coefficients to find moles produced.
\(\sf \large 3.\) Set up a ratio using the species of interest, like so (for a general reaction):

\(\sf \large \color{red}{a}A + \color{blue}{b}B \rightleftharpoons \color{green}{c}C\)

here upper case letters (A,B,C) are the chemical species, and lower case letters (a,b,c) are the coefficients,

\(\sf \dfrac{moles_A}{\color{red}{a}}=\dfrac{moles_B}{\color{blue}{b}}=\dfrac{moles _C}{\color{green}{c}}\)

You can use any pair from here, depending on what the question is asking for. For example: if you have 2 moles of B, how many moles of C can you produce? Now rearrange algebraically,

\(\sf\dfrac{2}{\color{blue}{b}}=\dfrac{moles_C}{\color{green}{c}}\rightarrow moles _C=\dfrac{2*\color{green}{c}}{\color{blue}{b}}\)


\(\sf \large 4.\) Solve

Answer 2

so first you need to balance the equation

nC2H4 + mO2 --> pCO2 + qH2O

HinT:- start with n = 2 and p = 4