A 15.0-mL sample of 0.100 M Ba(OH)2 is titrated with 0.125 M HCl. Calculate the pH for at least

A 15.0-mL sample of 0.100 M Ba(OH)2 is titrated with 0.125 M...

Key Concepts

Acid-Base Titration

An acid-base titration is an analytical technique used to determine the concentration of an acid or a base by reacting it with a standard solution of known concentration. The process involves gradual addition of one reactant to another until neutralization is achieved, making it a fundamental tool in chemical analysis and quantitative determination.

Equivalence Point

The equivalence point in a titration is the stage at which the number of moles of titrant exactly reacts with the number of moles of the substance being titrated, based on the stoichiometry of the reaction. It is crucial for identifying the complete neutralization of the analyte, and the pH at this point can provide important information about the nature of the acid-base reaction.

Reaction Stoichiometry

Reaction stoichiometry in titration involves the precise calculation of the mole relationships between the reactants. For example, in a titration involving a diprotic base and a monoprotic acid, the stoichiometric ratio indicates how many moles of acid are required to neutralize each mole of the base. Understanding these ratios is essential for accurately determining the equivalence point and for performing subsequent pH calculations.

pH Calculation in Titrations

Calculating pH during a titration involves determining the concentration of excess acidic or basic species present at different stages of the titration. Before the equivalence point, excess base or acid is present, creating a high or low pH respectively. At the equivalence point, the neutralization is complete, and the pH is determined by the properties of the resulting salt and any residual hydrolysis. Beyond the equivalence point, the concentration of the excess titrant governs the pH.

Titration Curve Analysis

A titration curve is a graphical representation of pH as a function of the volume of titrant added. It provides a visual display of the pH changes throughout the titration process, including buffer regions, the steep change at the equivalence point, and post-equivalence regions. Analyzing the curve helps in understanding the titration dynamics and identifying key points such as the starting pH, buffer region, equivalence point, and the effect of dilution.

Transcript

00:01 Okay, we have a 15 milliliter sample of 0 .1 molar barium hydroxide and it's titrated with 0 .125 molar hydrochloric acid.

00:07 Calculate the ph for at least five different points throughout the titration curve and sketch the curve.

00:12 Indicate the volume at the equivalence point.

00:15 So let's go ahead and find that volume at the equivalence point first because it asks for it.

00:22 So this one has two hydroxides versus just one hydrogen ion.

00:26 If i go ahead and calculate the moles for the base, we've got 0 .015 liters, change on the liters, times the molarity, will give us the 0 .0015 moles of the barium hydroxide.

00:46 Now because there's two hydroxides for every one barium hydroxide, i'm going to go ahead and multiply that by 2 to get 0 .0030 moles.

00:57 Of our hydroxide.

01:01 Now i know that at equivalence point the moles of hydroxide are equal to the moles of hydrogen ions and then i need to work backwards to go ahead and find the volume of hcl.

01:15 So if i have 0 .003 0 moles of hydrogen, i can divide by the molarity of the hydrochloric acid to find the liters and you get 0 .024 liters which is equivalent to 24 .0 milliliters.

01:36 We also know that at this point because it's a strong base titrated with a strong acid, the ph at this point is equivalent to 7.

01:44 So now let's find another point on our graph.

01:46 Let's find the initial ph.

01:49 We know initially it's only the barium hydroxide.

01:52 So again if we know that it's 0 .100 molar in barium hydroxide, we know that there are two hydroxide ions for every one barium hydroxide hydroxide.

02:05 So i know that my malarity is 0 .200 in the hydroxide ions because it completely dissociates.

02:11 I can take the negative log of that hydroxide ion concentration and that will give me the poh, which is 0 .70.

02:26 And if you subtract from 14, that will give you the ph, which is 13 .3.

02:35 So initially, the ph is 13 .3.

02:43 Okay, let's pick another point in our titration.

02:45 Let's see what the ph is after adding 12 milliliters of hcl.

02:50 I'm going to go ahead and grab the moles of hydroxide that we already calculated.

02:56 We've got 0 .003 moles of our hydroxide ions.

03:01 And i'm going to go ahead and calculate the moles of hydrogen ions.

03:06 So let's change the 12 mililiters into liters.

03:09 The molarity of the hydrochloric acid will give us the moles.

03:17 And that's equal to the moles of hydrogen ions because it's a strong b, or strong acid, so it completely dissociates.

03:27 And we have a lot more hydroxides than hydrogen ions.

03:30 So i'm going to subtract to find out how much extra hydroxides we have.

03:34 And we have 0 .015 moles of hydroxide in excess.

03:41 Now at this point, i've added 15 milliliters of our base so 12 milliliters of our acid.

03:48 So total, i have 0 .032 liters, or 32 millimeters.

03:56 And if i divide, i can find the molarity, which is 0 .0469 molar in the hydroxide ion.

04:02 I can go ahead and plug that into my poh equation, which is negative log of the 0 .069.

04:09 So my poh is equal to 1 .33...

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