Calculate the pH of a solution containing 2g of caustic soda per litre of water
To calculate the pH of a solution containing caustic soda, we need to determine the concentration of the hydroxide ions (OH-) in the solution. Caustic soda, also known as sodium hydroxide (NaOH), is a strong base. When it dissolves in water, it dissociates completely into sodium ions (Na+) and hydroxide ions (OH-).
Step 1: Convert grams to moles
To determine the concentration, we need to convert the mass of caustic soda into moles. The molar mass of sodium hydroxide (NaOH) is calculated as follows:
Na (sodium) = 22.99 g/mol
O (oxygen) = 16.00 g/mol
H (hydrogen) = 1.01 g/mol
Molar mass of NaOH = 22.99 g/mol + 16.00 g/mol + 1.01 g/mol = 39.99 g/mol
Given that there are 2 grams of caustic soda, we can calculate the number of moles:
moles = mass/molar mass = 2g/39.99 g/mol = 0.05 moles
Step 2: Calculate the concentration in moles per liter
Since the question states a concentration of 2g per liter, we know that 0.05 moles exist in a liter of water:
Concentration (in mol/L) = moles/volume = 0.05 moles/1 L = 0.05 M (Molarity)
Step 3: Calculate pOH
The pOH is the negative logarithm (base 10) of the hydroxide ion concentration.
pOH = -log[OH-]
In this case, the hydroxide ion concentration is 0.05 M. So, the pOH is:
pOH = -log(0.05) = 1.3
Step 4: Calculate pH
The pH is calculated using the formula:
pH = 14 - pOH
pH = 14 - 1.3 = 12.7
Therefore, the pH of the solution containing 2g of caustic soda per liter of water is approximately 12.7.