How many grams of HCl are needed to change 1000 L of water with a pH of 11 to a pH of 5.5?

To find out how many grams of HCl are needed to change the pH of water from 11 to 5.5, we need to calculate the amount of HCl required to neutralize the excess hydroxide ions and bring the pH down to 5.5.

To do this, we can follow these steps:

Step 1: Calculate the concentration of hydroxide ions in water at pH 11.
The pH scale is logarithmic, so each unit change in pH represents a ten-fold change in the concentration of hydrogen ions or hydroxide ions. The concentration of hydroxide ions (OH-) can be found using the pOH value, which is the negative logarithm of the hydroxide ion concentration.

pOH = 14 - pH
pOH = 14 - 11 = 3

Since the pOH is 3, the concentration of hydroxide ions is 10^-3 M.

Step 2: Calculate the concentration of hydrogen ions in water with a pH of 5.5.
The pH of 5.5 indicates that the concentration of hydrogen ions (H+) is 10^-5.5 M.

Step 3: Find the difference in concentrations between the initial and target solutions.
The difference in concentrations between the initial hydroxide ion concentration (10^-3 M) and the target hydrogen ion concentration (10^-5.5 M) is 10^-3 M - 10^-5.5 M.

Step 4: Calculate the moles of hydroxide ions to be neutralized.
Since concentration (M) = moles/volume (L), we can calculate the moles of hydroxide ions by multiplying the concentration difference by the volume of water.

Moles of hydroxide ions to be neutralized = (10^-3 M - 10^-5.5 M) * 1000 L

Step 5: Convert moles of hydroxide ions to moles of HCl.
Since HCl reacts with hydroxide ions in a 1:1 ratio, the moles of HCl required are the same as the moles of hydroxide ions to be neutralized.

Step 6: Calculate the mass of HCl using the molar mass of HCl.
The molar mass of HCl is approximately 36.46 g/mol.

Mass of HCl = Moles of HCl * Molar mass of HCl

By following these steps, you can find the precise amount of grams of HCl needed to change 1000 L of water with a pH of 11 to a pH of 5.5.