Chemistry Units and Ratios, Part 6: Converting Solution Mass to Solute Atoms; A Guided Example

Greetings,

This post is a continuation of my last post which included a Map Conversion Chart for Chemical Solution Problems.  As stated before, it's difficult to set up these kinds of multi-step conversion problems without a chart to follow.

Using a conversion chart, the beginning quantity and final quantity can be determined.  One then begins by writing the starting quantity and following the chart to determine the conversion factors to use.  Dimensional analysis is used, either in a step-wise fashion or all at once.  The presentation below provides a good example for an aqueous solution of methanol.  In this example, properties of the solution are provided and then a question is raised: How many hydrogen atoms are supplied by the methanol solute, given a certain mass of aqueous methanol solution?



You can run the presentation automatically by pressing the "play" button or you can advance the slides manually using the arrow button.  Please note that the final answer is rounded to three significant figures, because there are three sig. figs. in the provided mass of solution.  Also, note that each individual conversion answer (slides 5 to 8) is rounded to six sig. figs. in order to avoid a rounding error in the final result.  One could reason that the answer (number of hydrogen atoms) has an infinite number of significant figures (and therefore rounding concerns don't really apply), however the outcome of this problem is based on a mass measurement, which has a certain degree of precision directly dictating an acceptable variation limit of the H atom number result.  In other words, unless we are counting atoms directly (such as under an atomic force microscope), our number of H atoms result is a measurement and therefore is subject to significant figures rounding rules!

Thank you for reading!


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