Question

I need help plugging these in. I didn’t understand what DrBoB222 meant

2.A student starts with a titration by placing 30.0mL of 0.25M NaOH into a flask with 3 drops of phenolphthalein. The student then titrated to the endpoint using 18.0mL of HCI. Calculator concentration (molarity) of the HCI that the student used
NaOH + HCl ==> NaCl + H2O 
mols NaOH = M x L = ? 
The equation is 1 mol HCl to 1 mol NaOH; therefore, 
mols HCl = mols NaOH 
Then M HCl = mols HCl/L HCl = ?

Answer 1

For finding an unknown molarity of a substance in a titration, we use the following formula:

(M1 * V1) / (M2 * V2) = (n1 / n2)

Where,
M1 = Molarity of chemical 1
M2 = Molarity of chemical 2
V1 = Volume of chemical 1
V2 = Volume of chemical 2
n1 = Stoichiometric coefficient in the reaction of chemical 1
n2 = Stoichiometric coefficient in the reaction of chemical 2

You've been given M1, V1, and V2. Dr.Bob showed you, with the reaction, that n1 = n2 = 1. Now, you're left with just one variable, M2. Can you find the required Molarity?