Stoichiometry with Molar Concentration

Molar Concentration Recall that molar concentration is c = n/v We can find moles by rearranging the equation to be n = c*v So we can find the number of moles if we know the concentration and the volume of the substance

Note! If a volume is mentioned and the problem is about molarity, do not assume 22.4L as that is only for GAS at STP.

Example - 1 Barium hydroxide has a mass of 0.500g. What volume of HCl that is 0.250M is neutralized with this barium hydroxide? Ba(OH)2 + 2HCl → BaCl2 + 2H2O Neutralize for acid and base means the moles of acid = moles of base

Example - 2 What volume of CO2(g) at STP is produced if 2.5L of 0.150M HCl reacts with excess CaCO3?

Titration It is a process we use in chemistry to find the concentration of an acid or base We can perform this process when we know the exact volume and concentration of an acid or base and the volume of the substance we are neutralizing it We use a tool called a burette to perform this as it is highly accurate and easy to control the volume down to 0.1mL

Titration What we do We have an unknown substance in an Erlenmeyer flask with a known volume We add our known concentration substance into the burette We add an indicator into our unknown substance Phenolphthalein – indicator turns pink at around pH of 8.5 We slowly add or known substance into the flask and stop when our Erlenmeyer flask of solution is pink This is our equivalence point

Equivalence point or stoichiometric point This is the point in a titration where the moles of the acid and the moles of base meets the molar ratio of the balanced reaction Their mole ratio matches the coefficients Ba(OH)2 + 2HCl → BaCl2 + 2H2O So the mole ratio will match when you have twice as many moles of HCl present compared to Ba(OH)2 as their ratio is 2:1 and this is the equivalence point

Example - 3 H2SO4 + 2NaOH → Na2SO4 + 2H2O If an unknown concentration of H2 SO4 is in a flask and we add 0.800mol of NaOH for it to reach the equivalence point, how many moles of H2SO4 was in the flask? If an unknown concentration of 0.025L NaOH is in a flask and we add 0.500mol of H2SO4. What is the concentration of NaOH in the flask?

Note! Equivalence point is met when the moles are equal based on their molar ratio, not their concentration!

Example - 4 H2SO4 + 2NaOH → Na2SO4 + 2H2O It takes 21.55mL of NaOH at 0.500M to titrate 25.00mL of H2SO4. What is the molarity of H2SO4 present?

Example - 5 What volume of 0.450M KOH is required to react with 150mL of 0.300M H3 PO4 in order to produce a solution of K2HPO4? H3PO4 + 2KOH → K2HPO4 + 2H2O

Remember! In a titration, the equivalence point is when the molar ratios are met thus it is the comparison between moles, not concentration! 3A + 2B → 3C The equivalence point is met when we have any ratio of 3A to 2B.

Practice - 1 Page 131 - #17-25