Answers to Second Semester Exam Review Lab-Based Problems

Chemistry                                                                                                                                                     May 2007

Unit 7: States of Matter “Getting the Volume Right”

Task 1: Determine how much HCl and sodium carbonate she should use to produce 1.00 L of gas.  Clearly lay out the chemical equations and calculations.  Explain with a sentence or two what a set of calculations are determining.

Based on the stoichiometry and assuming 100 mL of HCl solution in which 88 mL of my CO2 will dissolve, I will need 5.04 g of sodium carbonate and 95.2 mL of 1 M HCl.  This was determined by using 1.08 L in the ideal gas law equation in order to determine the moles of CO2 I need to yield the 1.00 L volume I am aiming for.  In the actual experiment I will use a slight excess of the HCl.  I will use 110 mL in order to ensure that all the sodium carbonate reacts.

Task 2: Simply and clearly describe the procedural steps that the student needs to take to accomplish this task in the lab.  Point out any particular pitfalls that she should avoid.

[I leave the answer of this to you the student.]

 

Unit 6: Stoichiometry – Determining the formula of a New Salt

A student is analyzing a hydrated salt, containing Cr, Cl and H2O.  She must determine the empirical formula of the salt including the moles of water to salt ratio.  To do so she gets a clean, dry, crucible and weighs it.  She then puts about two grams of the salt into the crucible, weighs it again.  She meticulously heats the salt noting a complete change in color as the heating progresses.  After she is satisfied that she has thoroughly dehydrated the salt, she lets the crucible cool and weighs it.  She then dissolves the salt in a small amount of water and adds a large piece of Aluminum wire.  She allows the wire to completely react and notes that a new metal has formed on the bottom of the beaker and the aluminum wire has shrunk significantly.  She separates the new metal and filters it.  She rinses and allows the filtered metal to dry.  She records the following data during the course of the lab:

            Mass of empty crucible:                                  12.96 g

            Mass of crucible and hydrated salt:                14.97 g

            Mass of crucible and salt after heating:          14.60 g

            Mass of filter paper:                                          1.27 g

            Mass of filter paper and metal:                         1.81 g

Clearly and thoroughly lay out all calculations and steps for your solution to finding the empirical formula.

 Mass of water = 14.97 g - 14.60 g = 0.37 g

Mass of Cr metal = 1.81 g - 1.27 g = 0.54 g

mass of chlorine = [14.60 g - 12.96 g] - 0.54 g = 1.10 g

moles of water = 0.37 g / 18 g = 0.021 mol H2O

moles of Cr = 0.54 g / 52 g = 0.010 mol Cr

moles of Cl = 1.10 g / 35.45 g = 0.031 mole Cl

mole ratio 0.031 Cl / 0.010 mol Cr = 3 Cl : 1 Cr

mole ratio 0.021 H2O / 0.010 mol Cr = 2 H2O : 1 Cr

Empirical Formula:  CrCl3 - 2H2O

Answer the following questions.

  1.  Write the chemical equation for the reaction with aluminum.

            Al + CrCl3 --> AlCl3 + Cr 

 2.   What effect on the final complete empirical formula would not driving off all the water have had?  If all the water is not driven off this will make the mole of H2O too small and the moles of Cl too large.

Unit 10: Acids and Bases: Turn it Pink!  Acid Solution Stoichiometry

Questions:

  1. Add up the total mass of calcium you needed just before the solution turned pink.  What percentage of the total calcium mass is the “extra” you added to turn the solution pink?  [This is based on your data.]
  2. Give two reasons why your experimental mass of calcium added didn’t agree with your theoretical mass of calcium calculated.  Possible reasons: inaccuracy in the molarity, inaccuracy in weighing the Ca.  The calcium was not pure; there was an oxide coating.  This meant that one was not weighing out pure calcium but there was some oxygen in the mass of the weighed calcium.
  3. The reaction that caused the acid solution to turn pink is a different reaction than the equation that you wrote above in procedural step 1.  Write the equation of the reaction with calcium and whatever else to show this reaction.  In a couple of sentences describe what is happening when the solution turns pink.  When the solution turns pink the solution is becoming basic.  This means that all the acid is used up and some other reaction is occurring.  Since the HCl solution was in fact mostly water there must be a reaction between the calcium metal and water taking place when all the acid has reacted.  The reaction looks like: Ca + 2 H2O --> Ca(OH)2 + H2.