titration calculation: 35cm3 of Hydrochloric Acid reacts with 27.5cm3, 0.3mol/dm3 NaOH. What is the Concentration of the Hydrochloric Acid?

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Write out what you know:

HCL - VOL: 35 --- CONC: (x) --- MOL: (x)
NaOH - VOL: 27.5 --- CONC: 0.3 --- MOL: (x)
Realise what you can work out - Mol of NaOH in this case
Mol = Conc x (Vol/1000)
so Mol = 0.3 x (27.5/1000)
Mol = 0.00825
Now you have all the Info for one substance and most importantly the moles of a substance.
Mol Ratio is 1:1 unless the question says otherwise
Rearrange Mol = Conc x (Vol/1000)
to make Conc = Mol / (Vol/1000)
Conc = 0.00825 / (35/1000)
Conc = 0.235

0.24mol/dm³ = Conc of HCL