Chem 140: Stoichiometry Lab

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‎Stoichiometry Lab Procedure (PDF)

Prelab

In order to complete the calculations in this laboratory, it is required that you calculate the molar mass (also known as the formula weight or molecular mass) for three inorganic compounds. As always, please prepare your lab notebook by entering the title of the lab, your name, and the “Purpose” (enter the date and your lab partners name during the lab period). Make a table similar to the one below and complete the information; enter this information under the “Prelab” heading after the “Purpose” heading:

Next you will need to write out the chemical reactions under a heading of “Chemical Reactions.” The chemical reactions are written below in words. In your prelab you will write the chemical reactions as balanced chemical reactions, complete with the physical states [(s) – for solids, (aq) – for aqueous, (l) – for liquid, and (g) – for gas]. Example reaction: 2 Al (s) + 6 HCl (aq)  2 AlCl3 (aq) + 3 H2 (g) Reaction 1: Solid Na2CO3 is reacted with aqueous hydrochloric acid to form aqueous sodium chloride, liquid water and carbon dioxide gas. Balance this chemical reaction. Reaction 2: Solid NaHCO3 is reacted with aqueous hydrochloric acid to form aqueous sodium chloride, liquid water and carbon dioxide gas. Balance this chemical reaction. NOTE: This lab uses a concentrated form of HCl, a strong acid. Avoid skin contact and clean up any spills immediately. The HCl is dispensed in dropper bottles to minimize spillage, gloves are NOT recommended.

Purpose

The purpose of this lab activity is to investigate a chemical reaction with specific attention paid to the relationship between the mass of a substance and the moles of this substance. This relationship is generally referred to as the reaction stoichiometry. As a part of learning about reaction stoichiometry, you will also be introduced to balanced chemical reactions. The lab activity will introduce you to the use of the Bunsen/Tirrill burner and another opportunity to hone your skills in using the top-loading balance (PLEASE clean up any spills). It is again important to document all of your laboratory activities in your lab notebook.

Introduction

You will be given one sample labeled A or B. This sample is either sodium carbonate (Na2CO3) or sodium bicarbonate (NaHCO3). The goal of the experiment is to determine which compound you have been given. Both compounds react with HCl to form NaCl, water and carbon dioxide, but they form different amounts of NaCl. You can calculate the expected grams of NaCl using stoichiometry and the balance chemical reaction.

Experimental

- Measure the mass of a 250 mL Erlenmeyer flask to the nearest 0.01 g. Record the mass in your lab notebook. Place approximately 2 g of your unknown solid in the flask. You can use any amount of solid between 1.8 g and 2.2 g, but you need to know the exact mass of solid that you use. Record the combined mass of the flask and solid to the nearest 0.01 g. Record this mass in your lab notebook. - Return to your lab bench with your flask and unknown solid. Slowly add dropwise 3 M HCl into the unknown solid until no more CO2 (no more bubbles) is given off. Swirl the sample while adding HCl to make sure the contents is well mixed. - Place the flask on a ring stand as shown in lab-lecture. Gently heat the mixture with a small flame until only dry solid remains. It is possible that some material will splatter out of the flask if you heat too quickly, so be cautious. - Allow flask and NaCl to cool for ~10 minutes; when you can pick it up with your hand, it is cool enough. Determine the mass of the flask to the nearest 0.01 g and record your data in your lab notebook. - Clean and return all glassware to your drawer. - Once you have entered all data into your lab notebook, cleaned and rinsed all glassware with RO water, returned glassware to your drawer, then go to WebAssign and complete your “Reporting Sheet.”

Procedure: ‎Stoichiometry Lab Procedure