Bonjour a tous.
Je suis actuellement entrain de lire plusieurs protocole d'analyse iso. Et il y en a un qui me pose une difficulte de comprehension de calcul. C'est le protocol pour determiner la concentration d'une solution de HCl a l'aide d'une solution de cabonate de sodium.
Et le probleme est le suivant. Il prend pour masse molaire du carbonate de sodium 53g/mol. Or si nous la calculons avec le tableau periodique des elements nos obtenons 106g/mol. Au debut je pensais qu'ils effectuaient une simplification par 2. Mais le 2 reste dans la formule.
Cela est une reaction basique dont l'equation est :
2 HCl + Na2CO3 -> 2 NaCl + CO2 + H2O
Ainsi pourriez vous regarder et m'aider a comprendre. Merci.
Voici le paragraphe en question:
The exact molar concentration of 0.1 mol/ dm3 of hydrochloric acid solution is determined weekly using potentiometric determination of the end point of titration.
- Sample of HCl: 25.0 (± 0.1) cm3 (V9) of sodium carbonate solution ([Na2CO3] = 0.025 mol/dm3) is pipetted into the titration container and 75 (± 5) cm3 of distilled water is added. The container is placed on a magnetic stirrer, a magnetic stirrer coated with plastic and pH meter electrodes are lowered into the solution. The stirrer is turned on and stirred at a speed at which the whirlpool is barely noticeable. Titrate the sodium carbonate solution with a hydrochloric acid solution of 0.1 mol / dm3 concentration until the measuring device shows a pH of 4.5 ± 0.05. Register the volume of hydrochloric acid solution consumed for titration (V10).
-Blank: 100 (± 5) cm3 of distilled water as a blank sample is pipetted into the titration container. The container is placed on a magnetic stirrer, a magnetic stirrer coated with plastic and pH meter electrodes are lowered into the solution. The stirrer is turned on and stirred at a speed at which the whirlpool is barely noticeable. Titrate solution with a hydrochloric acid solution of 0.1 mol / dm3 concentration until the measuring device shows a pH of 4.5 ± 0.05. Register the volume of hydrochloric acid solution consumed for titration (V11).
The exact molar concentration of hydrochloric acid solution [HCl] mol/dm3, is calculated by the formula:
[HCl]=(2 ×m_Na2CO3 ×V_9)/(53,0 ×(V_10 - V_11) )
m_Na2CO3: the mass of sodium carbonate taken to prepare the solution , g
V_9: the volume of the sodium carbonate solution taken for titration is usually 25 cm3;
V_10: the volume of hydrochloric acid solution consumed for titration of sodium carbonate, cm3
V_11: the volume of hydrochloric acid solution consumed for titration of the blank sample, cm3
53.0 : molar mass of sodium carbonate (equivalent), g/mol
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