Video: Calculating the Final Buret Reading for a KOH-HCl Titration Based on the Initial Reading, the Volume and Concentration of KOH, and the Concentration of HCl

What is the final buret reading when a titration of 20.0 mL of 2.0 M KOH with 1.0 M HCl is performed if the initial buret reading was 4.65 mL? [A] 14.65 mL [B] 20.00 mL [C] 24.65 mL [D] 40.00 mL [E] 44.65 mL


Video Transcript

What is the final buret reading when a titration of 20.0 milliliters of 2.0-molar KOH with 1.0-molar HCl is performed if the initial buret reading was 4.65 milliliters? A) 14.65 milliliters, B) 20.00 milliliters, C) 24.65 milliliters, D) 40.00 milliliters, or E) 44.65 milliliters.

There’s quite a lot of information we need to unpick here. So let’s start by just recapping what a titration is. In a titration, we’ll probably have an Erlenmeyer or conical flask containing the material we’re going to analyze. In this case, we’ve got our 20 milliliters of 2.0-molar potassium hydroxide. How do we know it goes in the flask? Because we know its volume. The component we’re dispensing is the 1.0-molar HCl, and that’s in the buret. And the question tells us that the initial buret reading is 4.65 milliliters.

Remember in a buret that the zero point is at the top. So when we say that the initial buret reading was 4.65 milliliters, we just mean that the liquid level started at that point. And we’re going to go downwards to bigger numbers. When we perform the titration, we’re going to turn the tap and add some of the hydrochloric acid to the potassium hydroxide. We might have an indicator in there, like phenolphthalein, to help us detect the end point, the point where the hydrochloric acid and the potassium hydroxide have completely neutralized one another.

So where the question says “final buret reading” it means the reading on the buret once we’ve neutralized all the potassium hydroxide but not added any more HCl. So the final buret reading can be calculated by taking the initial reading and adding the minimum volume of 1.0-molar HCl required to neutralize 20 milliliters of 2.0-molar KOH.

In order to work that out, we’re going to need to know how KOH and HCl react. Potassium hydroxide is a strong base, while hydrochloric acid is a strong acid. So we can predict that they’ll react together to form a salt, in this case potassium chloride, and water. And they do this in a one-to-one ratio.

In order to calculate the volume of HCl required to neutralize our KOH, let’s start with the volume of our KOH, 20 milliliters. Now we want to convert that into moles of KOH by multiplying through by the concentration. But we’re dealing with different volume units, milliliters for our initial volume and per liter for our concentration. But we know that 1000 milliliters is equivalent to a liter. So we can multiply 20.0 milliliters by one liter per 1000 milliliters. If we calculated through at this point, we’d work out that we have 0.04 moles of potassium hydroxide. So let’s substitute that in just to keep it clean.

The next step is to convert from moles of potassium hydroxide to moles of hydrochloric acid. We know from the equation that it takes one mole of HCl to neutralize one mole of KOH. So we can convert to moles of HCl by multiplying by one mole of HCl per mole of KOH. If we evaluate this, we find that we need 0.04 moles of HCl to be added in the titration.

All that remains is to turn the amount of HCl via its concentration into its volume. We can do that by dividing through by the concentration of our solution, which is 1.0 molar or 1.0 moles per liter. This gives us a volume of 0.04 liters.

All that remains is to convert that into milliliters, which we can do easily by multiplying by 1000 milliliters per liter, which evaluates to 40 milliliters. You may have noticed that we converted from milliliters to liters and back. So a quick way would’ve just been to take the volume, multiply it through by the ratio, and then multiply it by the ratio of the concentrations. You would just have to be careful that you got the right terms in the right places.

Either way, we now know that we need 40 milliliters of 1.0-molar HCl to neutralize our KOH. So our final buret reading will be 40 milliliters lower down the buret, which is equal to 4.65 plus 40 milliliters, which is 44.65 milliliters. Our answer for the final buret reading when a titration of 20.0 milliliters of 2.0-molar KOH with 1.0-molar HCl is performed if the initial buret reading was 4.65 milliliters is 44.65 milliliters.

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