Mass % NaHCO 3 in Alka Seltzer In addition to this presentation, before coming to lab or attempting the prelab quiz you must also:  Review the video about use of the balances if needed  Read the introduction to the lab exercise in the coursepack

What’s the point? Experimentally apply reaction and formula stoichiometryExperimentally apply reaction and formula stoichiometry Introduce the idea of limiting reagentsIntroduce the idea of limiting reagents Chapter 3, Brown, LeMay and BursteinChapter 3, Brown, LeMay and Burstein Review the concept and use of mass %Review the concept and use of mass % Practice interpretation of graphsPractice interpretation of graphs

Background Alka-Seltzer medicine used to neutralize stomach acid by reaction with a basemedicine used to neutralize stomach acid by reaction with a base the basic component of Alka-Seltzer is NaHCO 3 (sodium bicarbonate)the basic component of Alka-Seltzer is NaHCO 3 (sodium bicarbonate) In the presence of acid (H + ), the reaction is NaHCO 3 (aq) + HCl(aq)  H 2 O( l ) + CO 2 (g) + NaCl(aq) Notice that a gas (CO 2 ) is generated. This makes the “fizz”Notice that a gas (CO 2 ) is generated. This makes the “fizz”

Reaction Stoichiometry NaHCO 3 (aq) + HCl(aq)  H 2 O( l ) + CO 2 (g) + NaCl(aq) For each mole of CO 2 (g) produced, 1 mole of NaHCO 3 reactsFor each mole of CO 2 (g) produced, 1 mole of NaHCO 3 reacts We say there is a 1:1 stoichiometric ratio between CO 2 and NaHCO 3We say there is a 1:1 stoichiometric ratio between CO 2 and NaHCO 3 Notice also that for each mole of CO 2 (g) produced, 1 mole of HCl must also have reactedNotice also that for each mole of CO 2 (g) produced, 1 mole of HCl must also have reacted There is a 1:1 stoichiometric ratio between CO 2 and HClThere is a 1:1 stoichiometric ratio between CO 2 and HCl

Example Calculation: Mass % mass percent NaHCO 3 in a tablet:mass percent NaHCO 3 in a tablet: % NaHCO 3 = (mass of NaHCO 3 / tablet mass) * 100% An Alka-Seltzer tablet was g. Reaction of the tablet yielded g of CO 2. What was the mass % NaHCO 3 in the tablet?

Alka-Selter tablet mass = gAlka-Selter tablet mass = g CO 2 mass = gCO 2 mass = g Hmm, we need the NaHCO 3 mass!Hmm, we need the NaHCO 3 mass! How is the CO 2 mass related to NaHCO 3 ?How is the CO 2 mass related to NaHCO 3 ? g x 1 mole CO 2 = mole CO g CO 2 To relate amounts of two chemicals in a reaction we use the stoichiometric ratio (i.e., the mole ratio in the chemical equation).To relate amounts of two chemicals in a reaction we use the stoichiometric ratio (i.e., the mole ratio in the chemical equation). We can relate the moles of CO 2 to moles of NaHCO 3We can relate the moles of CO 2 to moles of NaHCO 3 To get from g CO 2 to moles CO 2, use the molar mass.To get from g CO 2 to moles CO 2, use the molar mass.

CO 2 moles = molesCO 2 moles = moles Use stoichiometry to convert mol CO 2 to mol NaHCO 3 :Use stoichiometry to convert mol CO 2 to mol NaHCO 3 : NaHCO 3 (aq) + HCl(aq)  H 2 O( l ) + CO 2 (g) + NaCl(aq) NaHCO 3 (aq) + HCl(aq)  H 2 O( l ) + CO 2 (g) + NaCl(aq) For each mole of CO 2 (g), 1 mole of NaHCO 3 reactsFor each mole of CO 2 (g), 1 mole of NaHCO 3 reacts mol CO 2 x 1 mol NaHCO 3 = mol CO 2 mol NaHCO 3 1 mol CO 2 mol NaHCO 3 Convert moles CO 2 into moles NaHCO 3 by stoichiometry

NaHCO 3 moles = molesNaHCO 3 moles = moles To convert moles NaHCO 3 to g, use the molar mass.To convert moles NaHCO 3 to g, use the molar mass mole NaHCO 3 x g NaHCO 3 = 1.28 g 1 mole NaHCO 3 1 mole NaHCO 3 Given the mass of NaHCO 3 (1.28 g) and the tablet mass (3.234 g), we can find the mass % NaHCO 3 :Given the mass of NaHCO 3 (1.28 g) and the tablet mass (3.234 g), we can find the mass % NaHCO 3 : 1.28 g x 100% = 39.6% g Note: 100% is considered an exact number

Preparing Solutions by Dilution In this experiment, you must prepare 1M HCl by diluting stock 6 M HClIn this experiment, you must prepare 1M HCl by diluting stock 6 M HCl Remember, M = molar concentration (mol per L)Remember, M = molar concentration (mol per L) The Dilution Equation M 1 V 1 = M 2 V 2 M 1 = initial concentration (before dilution) M 2 = final concentration (after dilution) V 1 = volume of undiluted sample V 2 = volume of diluted sample

Dilution Calculation and Procedure How many mL of 6 M HCl are needed to prepare 250 mL of 1 M HCl by dilution? M 1 = 6 MV 1 = ? M 2 = 1 MV 2 = 250 mL We know our starting concentration, but not how much we need to use to get the final desired volume and concentration, so…

M 1 V 1 = M 2 V 2 (6 mol L -1 )(? L) = (1 mol L -1 )(0.250 L) V 1 = L So, if we dilute 41.7 mL of 6 M HCl to 250 mL with DI water, we will have prepared 1 M HCl. Dilution Calculation and Procedure Procedure measure 41.7 mL of 6 M HCl into a graduated cylinder measure 41.7 mL of 6 M HCl into a graduated cylinder transfer to a 250 mL volumetric flask transfer to a 250 mL volumetric flask dilute to the mark with DI water and mix thoroughly dilute to the mark with DI water and mix thoroughly

Limiting Reagent NaHCO 3 (aq) + HCl(aq)  H 2 O( l ) + CO 2 (g) + NaCl(aq) If you had 1 mol of NaHCO 3 but only 0.5 mol of HCl, you’d run out of HCl before all the NaHCO 3 reacted.If you had 1 mol of NaHCO 3 but only 0.5 mol of HCl, you’d run out of HCl before all the NaHCO 3 reacted. Then, HCl would be the “limiting reagent” (LR): the amount of CO 2 produced would be limited by the amount of HCl.Then, HCl would be the “limiting reagent” (LR): the amount of CO 2 produced would be limited by the amount of HCl. The moles of product obtained is is based on the moles of LR:The moles of product obtained is is based on the moles of LR: 0.5 mol HCl x 1 mole CO 2 = 0.5 mole CO 2 1 mole HCl 1 mole HCl

Safety lab goggles and coats must be onlab goggles and coats must be on use care in diluting the desktop 6M HCl to 1M HCluse care in diluting the desktop 6M HCl to 1M HCl HCl is a strong acid and is causticHCl is a strong acid and is caustic spills should be cleaned up immediately with copious amounts of waterspills should be cleaned up immediately with copious amounts of water contact with skin should be avoidedcontact with skin should be avoided in the event of contact, flush the area with lots of water and have someone notify the instructorin the event of contact, flush the area with lots of water and have someone notify the instructor