Procedures Move your mouse cursor over the hydrochloric acid (HCl) and sodium hy

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Procedures

Move your mouse cursor over the hydrochloric acid (HCl) and sodium hydroxide(NaOH) on the Materials shelf. You will see that the concentration of the sodium hydroxide is known to be 0.1 M while the concentration of hydrochloric acid is unknown.

Take a burette from the Containers shelf and place it on the workbench.

Add 50 mL of sodium hydroxide (NaOH) titrant. (50 mL is the capacity of a burette). Notice that it doesn't fill completely to the top mark. This is because there is space in the burette below the bottom mark. In addition, this is NOT the volume in the burette but rather this is your INITIAL READING. In the wet lab, you would need to let some of the solution flow through the stopcock in order to assure that the NaOH solution completely filled the volume of the burette down to the tip. You don't have to do that here.

Note the buret reading as your initial NaOH volume (mL). You can double click on the buret and select show close up to view the volume more closely.

Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench.

Add 10 mL of hydrochloric acid(HCl), unknown concentration, to the Erlenmeyer flask. Record the volume of HCL (mL).

Add 10 mL of water to the Erlenmeyer flask. This increases the total volume in the flask, making it easier to see the color change, but note that the value to use for the volume of hydrochloric acid HCl when calculating its concentration is still 10 mL.

Add 2 drops of phenolphthalein solution to the Erlenmeyer flask.

Move the Erlenmeyer flask anywhere on the base of the burette. The Erlenmeyer flask is connected to the burette so that liquid will drip from the burette into the Erlenmeyer Flask.

You are now ready to start the first coarse titration. Try to find the length of time required to click and hold the burette knob so that you deliver approximately 2 mL of sodium hydroxide from the burette to the Erlenmeyer flask. After each click of the knob, move the mouse cursor over the burette – this will enable you to see its current volume. To determine the amount of titrant delivered from the beginning until now, subtract the amount of liquid currently in the burette from the starting volume.

Continue to add the sodium hydroxide titrant in 2 mL increments. Each time, note the burette volume level. Note when the end point is passed (when the color of your solution changes). You now know between which two readings the endpoint occurred. For example, if you recorded 32 mL before the end point, but 34 mL was past the endpoint, record the 32 mL as your FINAL NaOH (mL). Calculate the volume of titrant that was added when 32 mL were delivered (subtract your INITIAL NaOH (mL) reading). You know that the FINE TITRATION can begin after approximately that amount of titrant.

Remove the Erlenmeyer flask from the burette and place them both in the Recycle Bin.

Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench.

Add 10 mL of hydrochloric acid, 10 mL of Water and 2 drops of phenolphthalein to the Erlenmeyer flask.

Place the Erlenmeyer flask at the base of a new burette.

Add 50 mL of sodium hydroxide titrant to the burette.

Add the initial large quantity of titrant, determined performing the coarse titration, (which in the example in #10, was 16 mL) so that you can begin with the fine titration. The solution in the Erlenmeyer flask should still be colorless.

Add sodium hydroxide from the burette drop-wise. This means adding a single drop at time which is done with single, short clicks on the black knob. You may decide to click and hold for short times, but in doing so you may miss the exact endpoint of the titration.

When the solution in the Erlenmeyer flask changes color, stop adding titrant. Record FINAL NaOH (mL) reading. Calculate the NaOH Delivered (mL) by subtracting the initial reading from the final reading.

In theory, we would repeat at least two more FINE TITRATIONS with fresh samples of HCl and full burets. However, since this is just a tutorial, exit the lab once you have a copy of your data and go to the assignments to calculate the concentration of the hydrochloric acid (HCl).

Is this correct so far? Need help answering question 2.

1. For your most exact titration, record the following:

2. Calculate the molarity of the HCl concentration from the equation (C stands for concentration and V stands for volume):

C(acid) = C(base) * V(base) / V(acid)

a
   Volume of NaOH solution in the burette at the start (mL): 50ml b
   Volume of NaOH solution in the burette at the end (mL):
  38.75ml c
   Volume of NaOH solution delivered to the flask (mL):
  
  31.34ml d
   Volume of HCl solution in the flask (mL)
  20.1ml

Explanation / Answer

C(base) base denotes NaOH = 0.1 M

V(base) means that is delivered to the flask = 31.34 mL

V(acid) means that is present in the flask to be neutralized by NaOH = 20.1 mL

So now using the above mentioned equimolar equation :

C(acid) = C(base) * V(base)/V(acid) ..........(A)

Substitute all the above values in equation (A)

C(acid) = 0.1 * (31.34 / 20.1) = 0.155 M

So molarity of HCl = 0.155 M

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