1. Let’s examine the figure to see what molecules are being represented.
2 C2H2 +
5 O2 
4 CO2 +
2 H2O
QUESTION:
Consider the pictorial representation of a chemical change shown below. Using the smallest set of whole numbers, the coefficient of C2H2 in the balanced chemical equation for this reaction is...
A. 1, B. 2, C. 3, D. 4
Consider the pictorial representation of a chemical change shown below. Using the smallest set of whole numbers, the coefficient of C2H2 in the balanced chemical equation for this reaction is...
1, B. 2, C. 3, D. 4
PAUSE
Let’s examine the figure to see what molecules are being represented.
According to the legend, a carbon atom is represented here by a filled circle.
HIGHLIGHT carbon in legend
and a hydrogen atom is represented by a small unfilled circle.
HIGHLIGHT hydrogen in legend
Therefore, this …
CALLOUT “C2H2” pointing to one of the C2H2 molecules in before picture
is a C2H2 molecule.
We have three of this before picture.
One…
HIGHLIGHT Two…
HIGHLIGHT Three…
In the after picture, we have only one left.
HIGHLIGHT.
This means that during the reaction, the amount of C2H2 decreases.
C2H2 is being consumed. It’s a reactant. In the pictures, we lost two molecules of C2H2.
CLICK
What else do we have?
According to the legend., large unfilled atoms are oxygen atoms...
HIGHLIGHT Legend
Therefore, these...
HIGHLIGHT five O2 molecules in before picture
are O2 molecules.
We have five of these before... and none after the reaction.
Therefore, O2 is a reactant. The pictures shows the loss five O2 molecules in this picture.
CLICK CLICK
Similarly, we should be able to identify CO2 as a product of this reaction.
On the left, we have no CO2 molecules. On the right, we have four CO2 molecules shown.
HIGHLIGHT CO2 molecules while counting
one...two...three four
Therefore, the picture shows the formation of four CO2 molecules.
We should also be able to identify H2O as a product of this reaction.
THere is no H2O molecule on the left, and there are two shown on the right.
HIGHLIHGT H2O molecules while counting
one...two...
Therefore, the picture shows the formation of two H2O molecules
Therefore, one possible balanced equation for this reaction is
2 C2H2
HIGHLIGHT
plus
5 O2
yields
4 CO2
2 H2O.
The coefficients are
CIRCLE coefficients as they are called out.
2..., 5..., 4..., and 2....
This is not the one of many ways of balancing the equation.
We can multiply or divide all the coefficients by the same number and still have a balanced equation.
For example, we can double all the coefficients, and still get have a balanced equation
CLICK In this case, the coefficients would be 4, 10, 8, and 4.
Or we can divide all the coefficients by 2
CLICK We still have a balanced equation with coefficients set to 1,
CALLOUT “implied 1” 5/2, and 1...
CALLOUT “implied 1”
This question is specifically asking for the coefficient of C2H2 is the equation with the smallest set of whole number coefficients.
HIGHLIGHT “smallest set of whole numbers” in the question
THe coefficients 2, 5, 4, and 2
HIGHLIGHT eqeuation at the top
are the smallest set of whole number coefficients. Why? Because their greatest common factor is 1.
Besides one, there is no number that you can use to divide all four coefficients to get a set of smaller whole numbers.
Therefore, the correct answer is B. The coefficient of C2H2 is 2, provided we use the smallest set of whole number coefficients.
Note that test makers are sometimes sloppy. If you see a question like this on a test and it doesn’t specifically specify
the smallest set of coefficients, assume that this is implied.
CLICK PAUSE END RECORDING
4 C2H O2  8 CO2 + 4 H2O
C2H /2 O2  2 CO2 + H2O

2. Video ID: © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08