Acids, Alkalis and Titrations: Answers

How can litmus paper be used to determine whether a solution is acidic or alkaline?

If blue litmus paper turns red, the solution is acidic. If red litmus paper turns blue, the solution is alkaline.

Acidic solutions have a pH between 1 and 6. A weakly acidic solution would have a pH of around 4-6.

An acid is something which releases hydrogen ions (H⁺) when it dissolves. An alkali releases hydroxide ions (OH^-) when it dissolves.

Calclate the moles of the substance with a known concentration. That's the sodium hydroxide which has a volume of 30 cm³ and a concentration of 0.5 mol dm^-3.
Moles = conc x vol
Concentration needs to be converted from cm³ into dm³ by dividing by 1000.
30/1000 = 0.03 mol of NaOH
Now we just need to check the ratios between sulfuric acid and sodium hydroxide by writing a balanced equation.
H₂SO₄ + 2NaOH --> Na₂SO₄ + 2H₂O
There is a 1:2 ratio between the sulfuric acid and the sodium hydroxide, so we will have half the number of moles of sulfuric acid compared to sodium hydroxide.
0.03 mol / 2 = 0.015 mol of sulfuric acid
Concentration = moles / volume
Concentration = 0.015 / (55/1000) = 0.27 mol dm^-3