I don’t understand how to predict the pH when 50mL, 70mL..... is added ak base r

Question

I don’t understand how to predict the pH when 50mL, 70mL..... is added ak base reaction een: HCl anel NH for starting point point lessthan 7 Mection point is more Calculating poin . When w Tuesday Problem: What is the pH curve for the addition of hydrochloric acid to a sample of sodium carbonate? pH Curves for Acid-Base Reactions Experimental Design: Small volumes of HClag) are added to a measured volume of a polyprotic base. After each addition, the pH of the mixture is measured and the data is graphed. Prelab Hypownesis sodium carbonate solution. 2. Calculate the volume required to reach the 2ad equivalence point. 3. Calculate the pH at the 2"d equivalence point 4. Choose the indicator you will use for this titration 1. Predict the pH when 0 mL, 50 mL, 71 mL, 100 mL, 130 mL of HCl(ag) is added to the Materials: 0.10 mol/L HCla) pH meter 250 mL Erlenmeyer 0.10 mo/L Na2COs(aq) buret 50 mL volumetric indicators funnel distilled water

Explanation / Answer

Hi, I hope you can calculate the pH at 0 mL addition. If not, let us move step by step forward:

1. Na2CO3 is a salt of weak acid and strong base which will hydrolyse in water and at 0.1 M concentration, the pH= 7 + 0.5 pKa + 0.5 log C. pKa for H2CO3 is 2.12 x 10-4. Hence pH of 0.1 M Na2CO3 will be 8.34.

2. Now the volume of Na2CO3 is not specified in the qustion. Let us assume it to be 50 mL.

Now millimoles (mmol) of Na2CO3 present : Molarity x volume in mL = 0.1 x50 = 5 mmol

i) When 50 mL 0.1 M HCl = 5 mmol HCl is added to this:

Since the reaction is : Na2CO3 + 2 HCl -> 2NaCl + H2O + CO2 ,

it will react with 2.5 mmol of Na2CO3 only to form water and NaCl, and 2.5 mmol Na2CO3 will remain unreacted. Since NaCl does influence the pH much you can calculate the pH using the net volume to be 100 mL now for 2.5 mmol of remaining Na2CO3 which corresponds to 2.5/100 = 0.025 M conc of Na2CO3 . The pH can be calculated using the previous expression.

Note that at each addition, the volume of total solution will increase which should always be taken in account while calcualting the concentration to obtain pH correctly.

Please share your feedback if this answer helps you solving this.

Thanks,

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