If 2.5 g of NaOH is dissolved in 253 mL of water, what is the pH of the solution?
mols NaOH = grams/molar mass
M NaOH = mols/L solution
(NaOH) = M NaOH
pOH = -log(OH-)
pOH + pH = pKw = 14. You know pKw and pOH, solve for pH.
To determine the pH of the solution, we need to know the concentration of hydroxide ions ([OH-]).
To find the concentration of hydroxide ions, we need to know the moles of NaOH in the solution. We can calculate the moles of NaOH by dividing the given mass by the molar mass of NaOH.
The molar mass of NaOH is the sum of the atomic masses of sodium (Na), oxygen (O), and hydrogen (H) since there is one atom of each in the formula unit of NaOH. The atomic masses of Na, O, and H are 22.99 g/mol, 16.00 g/mol, and 1.01 g/mol, respectively.
So, the molar mass of NaOH is:
(22.99 g/mol) + (16.00 g/mol) + (1.01 g/mol) = 40.00 g/mol
To calculate the moles of NaOH, we divide the given mass of NaOH by its molar mass:
moles of NaOH = (2.5 g) / (40.00 g/mol)
Now, we need to calculate the concentration of hydroxide ions ([OH-]). Since NaOH is a strong base, it completely dissociates in water, yielding one hydroxide ion (OH-) for every one NaOH.
Since the volume is given in milliliters (mL), we need to convert it to liters (L) first. There are 1000 mL in one liter, so the volume is:
253 mL ÷ 1000 mL/L = 0.253 L
The concentration of hydroxide ions is calculated by dividing the moles of NaOH by the volume in liters:
[OH-] = (moles of NaOH) / (volume in liters)
Finally, we can use the concentration of hydroxide ions to calculate the pOH of the solution. The pOH is the negative logarithm (base 10) of the hydroxide ion concentration:
pOH = -log[OH-]
Since pH + pOH = 14 for water at 25°C, we can find the pH by subtracting the pOH from 14:
pH = 14 - pOH
By following the above steps and plugging in the values, you can find the pH of the solution.