I have a lab question that is asking if the soda ash in our analysis was NaHCO3 instead of Na2CO3, how would the volume of acid added change from (a) the starting point to the phenolphthalein end point? (b) the phenolphthalein end point to the bromocresol green end point? (c) the original starting point to the bromocresol green end point?
I think all the parts would require less HCl to be added since NaHCo3 is more acidic (less basic) than Na2Co3?
I also have the same question for if it was replaced with NaOH...would all the parts require more acid to be added because NaOH is more basic than Na2CO3?
Thanks!
To determine how the volume of acid added would change if the soda ash in your analysis was NaHCO3 instead of Na2CO3, you need to consider the reactions that occur during the titration process.
(a) From the starting point to the phenolphthalein end point:
In this stage, the reaction occurs between the base (NaHCO3 or Na2CO3) and the acid (HCl). The balanced equation for the reaction between NaHCO3 and HCl is:
NaHCO3 + HCl → NaCl + H2O + CO2
The balanced equation for the reaction between Na2CO3 and HCl is:
Na2CO3 + 2HCl → 2NaCl + H2O + CO2
Comparing the two reactions, you can see that NaHCO3 reacts with 1 mole of HCl, while Na2CO3 reacts with 2 moles of HCl. Therefore, if the soda ash is NaHCO3 instead of Na2CO3, you would require less HCl (approximately half the volume) to reach the phenolphthalein end point.
(b) From the phenolphthalein end point to the bromocresol green end point:
At the phenolphthalein end point, all the NaHCO3 or Na2CO3 has been neutralized by the HCl present. Therefore, the reaction from this point onward is between the remaining HCl and the indicator (phenolphthalein initially and then bromocresol green). The indicator does not affect the stoichiometry of the reaction. So, the volume of acid added from the phenolphthalein end point to the bromocresol green end point would be the same, regardless of whether the soda ash is NaHCO3 or Na2CO3.
(c) From the original starting point to the bromocresol green end point:
Combining the information from parts (a) and (b), if the soda ash is NaHCO3 instead of Na2CO3, you would require less HCl (approximately half the volume) from the starting point to the phenolphthalein end point, and then the same volume of acid from the phenolphthalein end point to the bromocresol green end point.
Regarding your second question, if the soda ash was replaced with NaOH, the volume of acid needed would indeed be different due to the higher basicity (alkaline nature) of NaOH compared to Na2CO3.
(a) From the starting point to the phenolphthalein end point:
The balanced equation for the reaction between NaOH and HCl is:
NaOH + HCl → NaCl + H2O
As you can see, 1 mole of NaOH reacts with 1 mole of HCl. So, if the soda ash is NaOH instead of Na2CO3, you would require more HCl to reach the phenolphthalein end point.
(b) From the phenolphthalein end point to the bromocresol green end point:
Similar to the case when NaHCO3 or Na2CO3 is present, from the phenolphthalein end point to the bromocresol green end point, the volume of added acid would be the same regardless of whether the soda ash is NaOH or Na2CO3.
(c) From the original starting point to the bromocresol green end point:
Combining the information from parts (a) and (b), if the soda ash is NaOH instead of Na2CO3, you would require more HCl from the starting point to the phenolphthalein end point, and then the same volume of acid from the phenolphthalein end point to the bromocresol green end point.