It’s just like finding out your test score!!! 97 correct out of 100 questions = 97 x 100 = 97% 100 Total of “something” x 100 = % TOTAL
The chemical composition can be expressed as the mass percent of each element in the compound.
Example: Determine the percent composition of C 3 H 8. Assume you have one mole of the substance. 3mol C = 3(12.01g/mol) = 36.03 g 8mol H = 8(1.01 g/mol) = 8.08 g Total MW = 44.11 g
Example: Determine the percent composition of iron(III) sulfate. Iron(III) sulfate = Fe 2 (SO 4 ) 3 2mol Fe = 2(55.85g/mol) = 111.70 g 3mol S = 3(32.07g/mol) = 96.21 g 12mol O = 12(16.00g/mol) = 192.00 g Total MW = 399.91 g
Hydrated Compounds Some compounds exist in a “hydrated” state. Some specific number of water molecules are present for each molecule of the compound.
Dessicants You’ve probably noticed that some consumer goods contain a small packet labeled “Silica gel: Do not eat”. What’s that packet for, anyway?
As you know, many fragile goods can be easily damaged by moisture. The silica gel in each packet is used to soak up water from the atmosphere. This minimizes moisture that causes damage during shipping.
Many ionic compounds can be used to soak up water. Before they absorb water, they’re referred to as “anhydrous”, which means “without water”.
After they’ve soaked up the maximum amount of water, they’re called “hydrates”, making that water molecules are stuck to them
If you heat hydrates to very high temperatures, they “dehydrate”, meaning that the water is lost. Once all of the water is lost, these compounds are again referred to as “anhydrous”.
In this lab, we will be dehydrating the hydrate of magnesium sulfate. Using the data from this lab, you will determine – the percent composition – and the empirical formula of the hydrate.
A hydrate can be analyzed by driving off the water with heat. The remaining substance is called the Anhydrous salt… meaning “without water” Some hydrates are a color different than their anhydrous salt… Cobalt (II) chloride hexahydrate is pink Without water it is blue.
Example: A 7.0 g sample of hydrated calcium nitrate is heated to constant mass. Find the percent water in the hydrate and the formula for it.
4.9 grams remains after heating This is the anhydrous salt. How much water was driven off from the hydrate? 7.0 g sample of hydrated calcium nitrate - 4.9 g anhydrous salt = 2.1 grams of water
To write the formula Covert the mass of anhydrous salt to moles. Convert the mass of water to moles. Write the formula for the hydrate.
MgSO 4 * ?H 2 O 1. Heat the sample to drive off all water. 2. Mass the anhydrous compound. 3. mass of water = Mass of hydrate – mass of anhydrous salt 4. Convert these masses to moles. 5. Calculate the mole ratio between the compound and the water molecules.
Hydration Number Some molecules attach themselves to water molecules. This is done in set numbers, depending on the molecule. For example, copper sulfate attaches to 5 water molecules. We say its hydration number is 5 CuSO 4 5H 2 O (copper (II) Sulfate Pentahydrate)
Anhydrides A compound that is normally a hydrate and has lost its hydration water is said to be anhydrous and is called an anhydride. BaCl 2 2H 2 OBarium Chloride Dihydrate BaCl 2 Barium Chloride Anhydride -or- Anhydrous Barium Chloride
Finding the Hydration Number The hydration number can be conveniently found by heating the compound and measuring its mass loss. This mass loss is usually due to the hydration water molecules being driven off. For example… A 15.35 g sample of Strontium nitrate, Sr(NO 3 ) 2 nH 2 O, is heated to a constant mass of 11.45 g. Calculate the hydration number.
Mass Hydrate15.35g Mass Anhydride 11.45g Mass of Water (mass loss)3.90g Sample Data:
Calculations Moles Anhydride Moles Water Molar ratio… Hydration Number is 4, Sr(NO 3 ) 2 4H 2 O (Strontium Nitrate Tetrahydrate)