Empirical Formula Help [ENDORSED]

[005095564]

When trying to find the empirical formula, when do we divide the mole by the smallest # of moles calculated? I'm not really sure what the next step is after you find the amount of moles in an element of a compound (g/(g/mol))

[NRobbins_1K]

As an example to answer your question, let's say that you were asked to find the empirical formula for a compound containing 52.14% carbon, 13.2% hydrogen and 34.73% oxygen. It sounds like you know how to convert these percentages to molar amounts, so I'm going to skip that part and assume that you can get to the point where you calculate that this compound contains, in a 100 gram sample, 4.35 moles of carbon, 13.12 moles of hydrogen and 2.17 moles of oxygen. At this point, you divide by the lowest molar amount, because right now you are trying to discover the ratio of the elements in the compound in terms of whole numbers (the empirical formula). You already know that the ratio is 4.35 : 13.12 : 2.17, but we need this ratio in terms of integers, and the way to find that is to divide by the smallest number in the ratio. When we do this, we get 2 : 6 : 1, in other words, for every mole of oxygen atoms, there are 2 moles of carbon and 6 moles of hydrogen. This is the empirical formula, and we can rewrite it as C2H6O. If this isn't clear enough, reply and let me know and hopefully I can clarify.

[Brian_Ho_2B]

When trying to find the empirical formula, when do we divide the mole by the smallest # of moles calculated? I'm not really sure what the next step is after you find the amount of moles in an element of a compound (g/(g/mol))

First, find the mass percent composition (percentage of total mass for each element). Then take each percentage and make it as a number of grams (eg. if an element is 39.2% of the total mass, then let's say in a 100 g sample that there's 39.2 grams of that element). Repeat this process for each element. After that, divide each number by the molar mass to find the number of moles of each element. After this step, you divide the numbers by the smallest number, and then you multiply the formula by a scalar so that all numbers are whole numbers.

[Christineg1G]

After you find the amount of moles in each element, you divide the mole values you found by the smallest number of moles calculated. After this step, the outcomes of your division (which should be rounded to whole numbers, if possible) will determine the values for the empirical formula.
If your outcomes of the division were not whole numbers, multiply all the values by a specific whole number in order to have whole numbers. For example if you had an outcome of 1.33 you would multiply it by 3 because it's closest to the whole number 4.
Hope this helps!

[AGulati_4A]

After dividing each molar value by the lowest one in the group, you have to just multiply those values you get and make them hole numbers. these numbers would be the empirical subscripts for your answer

[Mariah]

What if you cannot multiply to get a whole number? If I have 2.66 for example, could I just round to 3?

[Chem_Mod]

What if you cannot multiply to get a whole number? If I have 2.66 for example, could I just round to 3?

Multiply each result by the same whole number until both equal a whole number (or at least within a couple hundredths). If your result ends in one or the following, multiply all results by same factor.
If your number ends in...
.25 --- multiply all by 4
.33 --- multiply all by 3
.50 --- multiply all by 2
.66 --- multiply all by 3
.75 --- multiply all by 4

An alternative approach would be to turn all your values into mixed fractions such as 0.5 is 1/2, 0.33 is 1/3 and so on. This will allow you to have an easier visualization of the common factor that you have to multiply everything by to obtain whole numbers.

Hope this helps! :)

[Mariah]

Thank you! This helps a lot. I did not know we could round if it was just a couple hundreths off.