Here’s a general strategy for determining empirical formulas. QUESTION: A sample of a compound contains 3.581x1022 C atoms and 1.436x1023 H atoms. What is the empirical formula of the compound? A. CH, B. CH4, C. C2H5, D. C3H4 General strategy: determine count-to-count (or mole-to-mole) ratio; reduce ratio to small whole numbers (first divide all counts by the smallest count; then, if necessary, find smallest multiplier to make all numbers very close to whole numbers) A sample of a compound contains 3.581x1022 C atoms and 1.436x1023 H atoms. What is the empirical formula of the compound? A. CH, B. CH4, C. C2H5, D. C3H4 PAUSE CLICK Here’s a general strategy for determining empirical formulas. You can determine empirical formulas if you have enough information to determine the relative numbers of the atoms of different elements in the compound. In other words, if you can get a count-to-count ratio for the atoms of the elements in the compound. Remember that moles are just group counts, so you can also use mole-to-mole ratio. In fact, in most cases, we use first determine the mole-to-mole ratio. In the case given here, we are given the actual individual atom counts... we don’t need to convert to moles. Once you have determined the count-to-count ratio, all you have to do is to reduce the ratio to a small whole number ratio. Here’s a table that shows how this strategy is implemented. From the question, we note that the compound contains 3.581x1022 carbon atoms HIGHLIGHT number in question and in table and also contains 1.436x1023 hydrogen atoms So, the carbon-to-hydrogen count ratio is 3.581x1022 is to 1.436x1023. A good strategy for reducing this ratio to a whole number ratio is to first divide both numbers by the smaller number. HIGHLIGHT “first divide all counts by the smallest count” In this case, 3.581x1022 is the smaller number. Dividing 3.581x1022 by 3.581x1022 gives us 1.000 HIGHLIGHT 1 and dividing 1.436x1023 by 3.581x1022 gives us 4.010. Note that we keep four significant digits here because we have four significant digits in our original numbers. Since both 1.000 and 4.010 are both very close to whole numbers,... we’re done. The carbon-to-hydrogen ratio in this case is 1-to-4. Therefore, the empirical formula is CH4. The correct answer is B. CLICK PAUSE END RECORDING Carbon Hydrogen 3.581x1022 1.436x1023 1.000 4.010

Video ID: 4-7-3 © 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