Stoichiometry Involving Molar Concentration By, Abby Leung & Peggy Fung Block A
What is Stoichiometry? • Stoichiometry is the relationships between the amount of reactants used in a chemical reaction and the amount of products produced by the reaction.
Confused when stoichiometry involves molar concentration? MOLARITY? MOLES?! STOICHIOMETRY!
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5 REALLY EASY STEPS 1. Write an Equation 2. Balance the Equation 3. Find the mole of the given quantity 4. Molar Ratio 5. Molarity- Use equation to Solve Your Question
FACTS YOU SHOULD KNOW • STP = 22.4 L/mol • Molar Concentration = Molarity c= n/v -or- Molarity =mole/volume (in L) • KEEP TRACK OF SIG FIGS DURING YOUR CALCULATIONS!
1. Write the Equation Put reactants on the left side of equation & products on the right side. Reactants Products
2. Balance Equation Make sure that each element has the same number of atoms on both sides!
3. Find mole of given quantity Convert given quantity to # of mol of quantity • If given quantity in L: use STP to convert • STP= 1 mol/ 22.4 L • If given quantity in particles: Use Avogadro’s Number to convert • 6.02 × 10^23
4. Molar Ratio • Use molar ratio to convert moles of given quantity to wanted quantity *Note: Coefficients in balanced equation represent molar ratio. • Molar ratio used in fraction form • E.g. 3:4 =3/4
Which mol quantity in molar ratio is the denominator? • Denominator: mole ratio of the given quantity • Numerator: mole ratio of wanted quantity
5. Molarity • Use step 4’s answer & molar concentration to answer question. • This step turns mol of a quantity to the answer of the question. • Molarity given in form: ? M (mol/L) • Depending on question, you may need to rearrange molarity equation. • c=m/v equals v=m/c equals m=cv
Let’s Try It Together! A student wants to put 50.0L of hydrogen gas at STP into a plastic bag by reacting excess aluminum metal with 3.00M sodium hydroxide solution according to the reaction. What volume of NaOH solution is required?
Step 1: Write out the Equation Hydrogen gas = H2 (g) since hydrogen is a diatomic molecule Al(s) + NaOH(aq) + H2O(l) NaAlO2(aq) + H2(g) reactants products
Step 2: Balance the Equation 2Al(s) + 2NaOH(aq) + 2H2O(l) 2NaAlO2(aq) + 3H2(g)
Step 3: Find mole of given quantity • Given quantity: 50.0L Hydrogen gas at STP. • Moles of H2 = 50.0L(1mol/22.4L) = 2.32 mol H2
Step 4: Molar Ratio • Moles of given quantity: 2.32 mol of H2 • Moles of wanted quantity: ? Mol of NaOH • 2Al(s) + 2NaOH(aq) + 2H2O(l) -> 2NaAlO2(aq) + 3H2(g) • 2 : 3 ratio (mol of NaOH: mol of H2) • 2:3 = 2/3 Mol(NaOH)= 2.32 mol H2 (2 mol NaOH/3 mol H2)= 1.49 mol NaOH
What volume of NaOH solution is required? • 1.49 mol NaOH (step 4 answer) • 3.00M sodium hydroxide (NaOH) solution (given) • Volume(v) = Mole(n)/ Concentration(c) • (Rearranged Molarity equation) • Volume of NaOH = (1.49 mol) / (3.00mol/L) = 0.496L NaOH
You Are Done! • Answer: 0.496L of Sodium Hydroxide (NaOH) solution was required. ALL IT TAKES IS THESE 5 SPECIAL STEPS!
Still having troubles remembering how to do these questions? NO WORRIES! We have a song to help you!
*Tune: ‘If I were a Boy’ by Beyonce Knowles If I were a mole even just for a day I’d get up every morning just to Do stoichiometry problems… I love those questions With molar concentration Except I always had a Secret to solving all these tricky ones. If I were a mole I could always understand How you could solve this question With five really ea-s-y steps Remember tha-at Molar Concentration Is c equals n over v It’s also called molarity And you can use STP to solve If I were a Mole Lyrics
If I were a mole The first big step is to Write out a proper equation With all the right element symbols Step two is to-oo Balance the equation both sides ‘cause all elements need to have same number of atoms on both sides If I were a mole I could always understand How you could solve this question With 5 very eas-s-y steps Now let’s move on For step three I would Use STP to change litres to mole Because you need moles for step 4
Use coefficients formula to get The molar ratio We are almost at the end Recall mol of first element And molar ratio To find mol of what you want Next use molarity to Find the volume in litres.