Chapter4

# Chapter4 - 1 SOLUTIONS Definitions solvent bulk material...

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1 SOLUTIONS Definitions solvent – bulk material used to dissolve substance solute – material dissolved in solvent Examples 1) salt water solvent – water solute – salt 2) dish water solvent – water solute – dish soap 3) engine coolant solvent – ethylene glycol solute – water Concentration - amount of solute per amount of solvent - molarity ( 29 Definition Molarity M molesof solute litersof solution mol L : = = - molarity can be thought of a conversion factor between volume and # of moles. - we’ll consider other concentration units in 1190 Molarity calculations Example: What is the molarity of a solution of NaCl that has 1.73 mol NaCl dissolved in 2.14 L of solution? Example: What is the molarity of a KBr solution when 114.8 g of KBr is dissolved in water to make 538.1 mL of solution? How many moles of solute are to be dissolved? - note the conversion of mL to L

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2 Example: How many grams of gold are in 3.75 L of 0.024 M solution of Au(NO 3 ) 3 ? We can convert between volume of solution and moles of solute using molarity as a conversion factor. DILUTIONS - making a solution of lower concentration from a solution of higher concentration Question: If a 5.0 M solution of NaCl is available, how does one make 250 mL of 2.0 M NaCl? 1) To make solution, we need to have the correct number of moles of NaCl. M mol V mol M V mol mol L L mol f f f f f f f = = = = 2 0 0250 050 . . . 2) Now consider how to get 0.50 mol NaCl from a 5.0 M solution. M mol V V mol M V mol mol L L i i i i i i i = = = = 050 50 010 . . . 3) Therefore, to make 250 mL of 2.0 M NaCl, one needs 100 mL of 5.0 M NaCl. Then add water to make 250 mL solution. Reconsider step 2. Where did # of mol i come from? **Number of moles of solute is the same before and after dilution.** i. e., mol f = mol i Knowing mol = M V yields the dilution equation M f V f = M i V i For any concentration unit, the dilution equation can be written as c f V f = c i V i c – concentration
3 Example: Find the volume of 6.02 M HCl to make 500 mL of 0.125 M HCl. Lab Safety Aside: Never add water to acid, always add acid to water. THE AQUEOUS PHASE Q: What happens to salt when it is dissolved in water? - It is a physical change but how can that be? A: i) Solid salt is a collection of ions, Na + and Cl - , assembled in a lattice. - Ions are surrounded by each other. ii) When dissolved, the ions are surrounded by water. - Thus, the ions themselves don’t change, only their environment. Ions (or sometimes molecules) that are surrounded by water are in the aqueous phase . Ionic compounds which are soluble (that is, dissolve in water) dissociate (that is, break apart) into dissolved cations and anions. Examples: NaCl (aq) = Na

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## Chapter4 - 1 SOLUTIONS Definitions solvent bulk material...

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