Empirical formula and moles
By
Heidi Maloy
Created 3 years ago
Duration 0:11:38
201
How to find an empirical formula using grams and moles

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chemistry

Slide 1  How to work with moles
 And find an empirical formula with mole information

Slide 2  Class recap leading to solving Problem 3 on the worksheet Moles and empirical formulas
 OK so in class today for those of you who were at school we looked at the relationship between moles and grams and the # of things in a mole.

Slide 3  Dimensional Analysis
 Dimensional analysis is focused on units. You are looking for an answer in a specific unit and you use conversion factors to allow you to get to the desired unit.
 For example…how many atoms are in 10 grams of FE?

Slide 4  Problem # 3 on the worksheet
 3. What mass of water would you need to have 15.0 moles of H2O?

Slide 5  OK applying this mole knowledge to empirical formulas
 An empirical formula is the simplest formula that a compound can have. Not all formulas are empirical in nature, formulas are molecular and we will get to those later.
 The subscript ratio of any empirical formula is the based on the relative mole ratio between the elements in the compound.
 Let’s look at problem # 4 to see what this means:

Slide 6  Problem #4
 A compound was analyzed and found to contain 9.8 g of nitrogen, 0.70 g of hydrogen, and 33.6 g of oxygen. What is the empirical formula of the compound?
 Step 1 – find the # of moles of each element
 Step 2 – find the relative mole ratio
 Step 3 – write the empirical formula

Slide 7  Recap what do I need to know to do to find and empirical formula
 1. Find the # of moles of each element in the formula by dividing the mass of the element by its AMU.
 2. Divide all element moles by the element with the smallest # of moles.
 3. Round to the nearest whole number (unless the number is close to a .5)
 The relative mole ratio becomes the subscripts of your compound.