Determining Empirical and Molecular Formulas Chemtivity

Part 1: Empirical and Molecular Formulas

  1. A given compound has 1 carbon atom for every 2.5 hydrogen atoms. According to the atomic theory, atoms only combine in whole number ratios. What is the whole number ratio of carbon to hydrogen in this compound?
  1. A compound has the formula C4H10, does this have the same ratio of carbon to hydrogen as the compound in #1?
  1. Based on your answer to number 2, can you say that the two compounds described in 1 and 2 are the same compound? Why or why not?
  1. The empirical formula for a compound gives the lowest whole number ratio of atoms in a formula. C2H5 is an empirical formula of C4H10. Determine the empirical formula for the following compounds:
  2. Al2Cl6
  3. C2H6
  4. C6H12O6
  5. C2H6O
  6. H2SO4

Model 1: Some representations of ionic crystals:

Some representations of molecular substances:

waterethane glucoseribose

  1. When we write the formulas for ionic compounds we always give the empirical formula. Using the illustrations to help you, explain why we do this?
  1. What is the chemical formula for ribose? ______
  2. What is the chemical formula for glucose? ______
  3. What is the empirical formula for glucose? ______
  4. What is the empirical formula for ribose? ______
  5. Why is it necessary to describe glucose using the chemical formula? In other words, why can’t we use the empirical formula for molecules like glucose?

Part 2: How to determine the empirical formula from data

Through the next series of questions, you will experience the thought process that chemists might have used to determine just what the chemical formulas were. Keep in mind that you will need to repeat this process in the future when I give you experimental data and ask you to determine chemical formulas!

Before we begin, review some simple concepts:

  1. What is the chemical formula of iron (III) phosphate ______
  1. What is the ratio of Fe atoms to O atoms? (show as a fraction and a decimal) ______
  1. What is the ratio of P to O atoms? (show as a fraction and as a decimal)______
  1. What is the ration of Fe to P atoms? ______
  2. If …

a)a given sample of iron (III) phosphate has 10 Fe atoms, how many P atoms are there?

b)a given sample of iron (III) phosphate has 120 O atoms, how many P atoms are there?

c)a given sample of iron (III) phosphate has 1 mole of P atoms, how many Fe atoms are there?

d)a given sample of iron (III) phosphate has 0.5 moles of P atoms, how many Fe and how many O atoms (in moles) are there?

  1. How does the mole ratio of one element to another compare to the atom ratio? In other words, is the ratio moles of Fe atoms : moles of O atoms different from Fe atoms : O atoms?
  1. What is the molecular weight of iron (III) phosphate?
  1. What is the percent composition of each element in iron (III) phosphate? (show at least two decimal places)

Information:

Now, let’s say that you have managed to do some experiments in which you have decomposed (broken down) a sample of iron (III) phosphate and obtained the following data:

Table 1: Resulting Mass from the Decomposition of Iron (III) Phosphate

Mass ( 0.01 g) / Percent of sample
Original Mass of the Sample / 25.00 / n/a
Mass of the iron produced / 9.26
Mass of phosphorus produced / 5.14
Mass of oxygen produced / 10.61

Assumptions:

a)To determine the ratio of elements, we need to know how many atoms of each element are present.

b)We don’t really need to know the number of atoms, we just need the number of moles since moles is a way of counting large numbers of atoms

  1. Use the data to fill in Table 1. Show your work below.
  1. Did you get the similar results in 9 as you did in 8? ______
  1. What law allows you to assume that given any sample of iron (III) phosphate, the percent composition of the elements will be the same? *This law is true for ionic compounds, but not covalent.

In order to determine the formula, you need to know the ratio of atoms (right?). How can you calculate the number of atoms from the percent composition? In question 12-16 you will learn to do this.

  1. Assume, you had 100 grams of iron (III) phosphate and use your answer the %’s in #9 to show how you could determine the number of grams of each element.
  1. Once you know the number of grams, it is necessary to determine the number of atoms in order to find the ratio of the elements. Is it necessary to calculate the number of atoms, or can you just calculate the number of moles?
  1. Calculate the number of moles of each element from your answer to #12
  1. Find all the ratios for one element to another (Fe to O, P to O and Fe to P). First, divide the numbers of moles of Fe by the moles of O and write down the decimal. Convert this to a fraction. You will need to do some estimating. See the chart below for help. Do not use Math Frac on your graphing calculator! Repeat for the other ratios.

  1. How do these ratios compare to the ratios you wrote in 2, 3 and 4?

Practice

  1. The mole ratio of Carbon to hydrogen in a given compound is 0.333, write the empirical formula.
  2. The mole ratio of carbon to hydrogen in a given compound is 0.749, write the empirical formula.
  3. Another sample of an iron compound is found to be 46.866% Fe, 17.329% P and 35.085% O what is the chemical formula of this compound? (Show your work!)
  4. Assume you have 100 g, find the mass of each element
  5. Find the moles of each element
  6. Calculate the ratio
  7. Determine the formula

Part 2: How it use to be…

Once upon a time no one really knew about ionic compounds or that ions even formed. No one knew that Iron (III) had a +3 charge (it wasn’t always called iron (III)) and that a phosphate ion was made up of one phosphorus atom and one 4 oxygen atoms. They did know the properties of iron (III) phosphate; although, at the time it probably didn’t have this name. Scientists, being organizers of information and looking for patterns they can use to make predictions and better understand the world, thought that it would be useful to find out what atoms made up each substance and how many of each atom made up each substance. All of this had to be worked out from experiments.

Watch the video on the discovery of oxygen from Angel.

  1. Make a blog entry on Angel to answer the questions below. You can write it just as you imagine it, or you could pretend you were a scientist writing to another scientist to show the limitations of what they were able to do.

As a group, imagine what it must have been like to be a scientist in a time in which not all element were known, that the idea of molecules and chemical formulas was unknown even to scientists. What must it have been like to try to understand why the world works the way it does? How could they describe chemical and physical changes? How could they look for patterns? What mistakes could have been made? How difficult was it to communicate scientific findings if there was not systematic way for naming or identifying elements in compounds?

  1. At the end of the video the hosts describe how finding the element oxygen was the beginning of much of the chemistry we rely on today. Individually, in your notebook, explain what they meant by that.