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6-16: Analysis of Baking Soda

In this video we'll show you how to use the balance:

In this video we'll show you how to add indicators:

In this video we'll show you how fill the buret:

In this video we'll show you how to operate the buret:

Titrations provide a method of quantitatively measuring the concentration of an unknown solution. In an acid-base titration, this is done by delivering a titrant of known concentration into an analyte of known volume. (The concentration of an unknown titrant can also be determined by titration with an analyte of known concentration and volume.) Titration curves (graphs of volume vs. pH) have characteristic shapes. The graph can be used to determine the strength or weakness of an acid or base. The equivalence point of the titration, or the point where the analyte has been completely consumed by the titrant, is identified by the point where the pH changes rapidly over a small volume of titrant delivered. In this assignment, you will determine the mass % of an unknown sample of baking soda (NaHCO3) by titrating it with an HCl solution of known concentration.

 

  1. To start this activity, click this link for Analysis of Baking Soda. The lab will load in a new tab. Click back to this tab to read further instructions and complete the questions below. You can follow along with the instructions below in the Procedures tab in the lab.
  2. Place the beaker in the spotlight close to the balance. Move the bottle of impure solid NaHCO3 to the spotlight next to the balance also. Click on the Balance area to zoom in, open the bottle by clicking on the lid (Remove Lid). Drag the beaker to the balance to place it on the balance pan and tare the balance. Click on the bottom image on the front of the bottle (the largest scoop) and that will load the scoop with that amount.  Drag the scoop to the beaker on the balance until it snaps in place and then let go. Repeat with a scoop from the 2nd from the bottom image (approximately 0.5 g) and put that into the beaker. Record the total mass of the sample in the data table below. Add 25 mL of water with a graduated cylinder. Add methyl orange indicator and the calibrated pH meter probe.
  3. The buret will be filled with 0.3015 M HCl. Click the Save Data button in Live Data so the titration data will be saved. The horizontal position of the orange handle is off for the stopcock. Open the stopcock by pulling down on the orange handle. The vertical position delivers solution the fastest with three intermediate rates in between. Turn the stopcock to one of the fastest positions. Observe the titration curve in Live Data. When the blue line begins to turn down, double-click the stopcock to turn it off. Move the stopcock down one position to add volume drop by drop.

There are two methods for determining the volume at the equivalence point:  (1) Stop the titration when a color change occurs. Click the Stop Saving button in Live Data and open the Lab Book to see the data table with all the titration data. Record the volume at the equivalence point in the data table below. OR (2) Add drops slowly through the equivalence point until the pH reaches approximately 12. Click the Stop Saving button. Open the Lab Book and click the Copy Data button to the top right of the data table to copy and paste the data to a spreadsheet. Plot the first derivative of pH vs. volume. The peak will indicate the volume at the equivalence point since this is where the pH is changing the most rapidly as the volume changes.

Data Table

mass unknown sample (g) volume HCl (mL)

  1. Write a balanced chemical equation for the reaction between NaHCO3 and HCl.

  1. Calculate the moles of HCl by multiplying the volume of HCl in liters and the molarity of HCl in mol/L. (Keep four significant digits in all of the calculations.)
moles HCl molarity HCl (mol/L)

  1. The moles of HCl can be converted to moles of NaHCO3 using the coefficients from the balanced equation. What is the mole to mole ratio of HCl to NaHCO3?

mol HCl : mol NaHCO3

  1. How many moles of NaHCO3 are present in the sample? mol

  1. Calculate the grams of NaHCO3 by multiplying the moles of NaHCO3 by the molecular weight of NaHCO3 (84.007 g/mol).

g


  1. The mass % of NaHCO3 present in the sample can be calculated by dividing the mass of NaHCO3 from question #8 by the mass of the sample from the data table and multiplying by 100.

 

What is the mass % of NaHCO3? %

Open the Unknowns tab in the tray, expand the Assigned Unknowns menu and enter your calculated mass %. Click Submit. Report the actual result that the program tells you you had here. That is also recorded in your LabBook.

Actual result: % NaHCO3