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Source: Smaa Koraym at Johns Hopkins University, MD, USA
In this experiment, you will create a saturated solution of sodium tetraborate decahydrate, also called borax. In water, borax dissociates into two sodium cations and one tetraborate anion. When an aqueous borax solution is saturated, it means that it contains the maximum amount of dissolved solute, which is borax, for that volume of solvent, which is water. Any additional solid won't appear to dissolve.
Since solubility is temperature-dependent, each group will create a saturated solution at a different temperature. More borax will dissolve at higher temperatures, resulting in a solution with a higher borax concentration.
|Temperature (°C)||Amount of borax (g)|
|Assigned temperature (°C)|
|Borax mass (g)|
|Trial||Solution temperature (°C)||Volume (mL) of 0.5 M HCl added|
Now you will determine how much borax has dissolved in your saturated solution. Recall the chemical reaction showing how borax dissociates in water, forming the tetraborate ion. Since the tetraborate ion is a base, it will react with acid following a neutralization reaction.
When the amount of acid is twice the amount of tetraborate, the solution is neutralized. To do this, we will slowly dispense HCl into the borax solution until it is neutralized, meaning that the acid and base react to form water and salt and a neutral pH. We'll use the pH indicator bromocresol green to let us know when the solution is neutralized, as it turns from blue to pale greenish-yellow when the pH is neutral.
|Assigned temperature (°C)||Tavg (°C)||Vavg (mL)||Tavg (K)||1/Tavg (K)||Moles of 0.5 M HCl||Molarity of Na2[B4O5(OH)4]||Ksp||lnKsp||ΔG (kJ/mol)|