In: Chemistry
A 360.0?mL buffer solution is 0.160M in HFand 0.160M in NaF.
What mass of NaOH could this buffer neutralize before the pH rises above 4.00?
If the same volume of the buffer was 0.370M in HF and 0.370M in NaF, what mass of NaOHcould be handled before the pH rises above 4.00?
given 360 ml of 0.160 M NaF and 0.160 M HF
moles of NaF = molarity x volume /1000
moles of NaF = 0.16 x 360 /1000
moles of NaF = 0.0576
moles of HF = 0.0576
Let moles of NaOH be y .
NaOH + HF -----> NaF + H20
from the reaction we get
1 mole of NaOH reacts with 1 mole of HF to give 1 mole of NaF
so y mole of NaOH reacts with y mole of HF to give y mole of NaF
new moles of HF = 0.0576 - y
new moles of NaF = 0.0576 + y
according to hasselbach henderson equation
we get
pH = pKa + log [ salt /acid]
pH = pKa + log [ NaF / HF]
4 = 3.17 + log [ 0.0576 + y / 0.0576 - y ]
y = 0.04275
mass of NaOH = moles x molar mass
mass of NaOH = 0.04275 x 40
mass of NaOH = 1.71 g
so 1.71 g of NaOH is needed
given 360 ml of 0.370 M NaF and 0.370 M HF
moles of NaF = molarity x volume /1000
moles of NaF = 0.37 x 360 /1000
moles of NaF = 0.1332
moles of HF = 0.1332
Let moles of NaOH be y .
NaOH + HF -----> NaF + H20
from the reaction we get
1 mole of NaOH reacts with 1 mole of HF to give 1 mole of NaF
so y mole of NaOH reacts with y mole of HF to give y mole of NaF
new moles of HF = 0.1332 - y
new moles of NaF = 0.1332 + y
according to hasselbach henderson equation
we get
pH = pKa + log [ salt /acid]
pH = pKa + log [ NaF / HF]
4 = 3.17 + log [ 0.1332+ y / 0.1332- y ]
y = 0.0988
mass of NaOH = 0.098 x 40
mass of NaOH = 3.955
so 3.955 g of NaOH is needed