Concentration Calculations

Concentration Calculations

Working through the steps

Why is Ms. Mendelsohn making us do these calculations!?!?!

The perfect cup of coffee (or tea) is very important. Which the next video to learn what coffee has to do with it.

Working through a word problem

Get out your science notebook and something to write with.

Pb(NO₃)₂ + NaOH -> ions and a ppt

Complete the chemical equation

Multiple Choice

What were your products?

• NO₃^- + Na⁺ + Pb(OH)₂

• NO₃^- + Na⁺ + PbOH

• NaNO₃ + Pb(OH)₂

You are informed that 42 mg of Pb(OH)₂ precipitate formed, calculate the starting molarity of Pb(NO₃)₂ in 50 mL of solution.

What is this telling us?

• We have 42 mg of solid Pb(OH)₂ formed in a 50 mL solution

• This is a concentration of mg/mL

• We need to find the number of moles in the 50 mL

42 mg of Pb(NO₃)₂ needs to be converted to something usable. Remember our goal is moles.

How will we do this?

Multiple Choice

To convert your 42 mg of Pb(NO3)₂ to moles, you should first convert your mg to g. What is the correct set-up?

• 
  

  $42\ mg\ \times\frac{1000\ \ mg}{1\ g}$

• $42\ mg\ \times\frac{1\ g}{1000\ mg}$

• $\frac{42}{1000}$

• $42\ \times1000$

Now look back at your original equation.

Pb(NO₃)₂ + NaOH -> ions and a ppt.

We have 42 mg Pb(NO₃)₂.

What is the amount of Pb(NO₃)₂ in grams?

Convert your grams of Pb(NO₃)₂ to moles

• Find your molar mass from the periodic table

• Set-up your calculation to convert your grams to moles using the molar mass

Multiple Choice

What is the molar mass of $Pb\left(NO_3\right)_2$  ?

•  $\frac{331.208\ g}{mol}$ 
  

• 241.214 mol

•  $\frac{331.2\ g}{mol}$ 

• 331.2 mol

Now you have the molar mass, AND you have your mass of Pb(NO₃)₂ in grams.

How many moles do you have?

Multiple Choice

How many moles of Pb(NO₃)₂ are in 0.042 g ?

• 0.000127 mol

  (aka 1.27 x 10^-4 mol)

• 0.0126 mol

  (aka 1.26 x 10³ mol)

• 126 mol

  (aka 1.26 x 10² mol

Looking back:

Given that 42 mg of Pb(OH)₂ precipitate formed, calculate the moles of Pb(NO₃)₂ in 50 mL of solution.

You have your 0.000127 mol of Pb(NO₃)₂ and now you need to put it in 50 mL of solution.

Convert your units so that you have the proper units of moles/Liter

You currently have mol/mL

Multiple Select

What is a good set-up for converting mol/mL to mol/L?

• $\frac{mol}{mL}\ \times\ \frac{1000\ ml}{1\ L}\ =\ \frac{1000\ mol}{L}$

• $\frac{mol}{mL}\times\frac{1\ L}{1000\ mL}\ =\ \frac{mol\cdot L}{1000\ mL^2}$

Calculate the final molarity of your Pb(NO₃)₂

Multiple Choice

Calculate the concentration of Pb(NO₃)₂ in your solution.

• 0.00254 M

• 0.0254 M

• 0.0827 M