You are titrating a solution of sodium hydroxide of unknown concentration with 0.500 M of phosphoric acid. It required 16.4 mL of H3PO4 solution to neutralize 10.0 mL of NaOH solution. Calculate the molar concentration of sodium hydroxide solution?

1 Answer
Tom M.
May 12, 2018

#2.46" mol dm"^-3#

Explanation:

#1mL=1cm^3# as a note.

Let's write our equation out first.

#3NaOH+H_3PO_4 -> Na_3PO_4+3H_2O#

We'll refer back to this later.

Start with the #H_3PO_4#

#"vol"=16.4"cm"^3#
#"conc"=0.500"mol dm"^-3#

Since #"conc"="moles"/"volume"#

#"moles "H_3PO_4=0.500xx16.4xx10^-3#
#=0.0082 "mol"#
We multiply by #10^-3# to convert the volume from cm3 into dm3.

From our equation:

#3NaOH+H_3PO_4 -> Na_3PO_4+3H_2O#

For one mole of phosphoric acid, we need three moles of sodium hydroxide. Therefore:

#"moles "NaOH=3xx0.0082#
#=0.0246" mol"#

Finally:

#"conc"="mol"/"vol"#

#"conc "NaOH=0.0246/(10xx10^-3#
#=2.46" mol dm"^-3#

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