What is the concentration of a solution obtained by diluting 30 mL of 15.8 M hydrochloric acid so the total volume of the solution will be 400mL

the concentration is inversely proportional to the volume

30 / 400 = c / 15.8 M

Also, there is a dilution formula that can be used.

mLa x Ma = mLb x Mb
30 x 15.8 = 400 x M
Solve for M.
Finally, a different play on what R_Scott wrote.
When you dilute a solution you KNOW it will get weaker, so start with the 15.8 and use a fraction arranged to get a smaller number like this.
15.8 x (30/400) = ?.You know to put the 30 on top and the 400 on the bottom because that's the only way you can get a smaller number. If you used 15.8 x (400/30) you will get a larger number and you know that can't be right.

To find the concentration of the solution obtained by diluting 30 mL of 15.8 M hydrochloric acid, you can use the concept of dilution formula. The dilution formula is given by:

C1V1 = C2V2

Where:
- C1 is the initial concentration of the solution (in this case, 15.8 M)
- V1 is the initial volume of the solution (in this case, 30 mL)
- C2 is the final concentration of the solution (unknown)
- V2 is the final volume of the solution (in this case, 400 mL)

Here's how you can plug in the values and solve for C2:

C1V1 = C2V2

(15.8 M)(30 mL) = C2(400 mL)

474 = C2(400)

To find C2, divide both sides of the equation by 400:

C2 = 474 / 400

C2 = 1.185 M

Therefore, the concentration of the solution obtained by diluting 30 mL of 15.8 M hydrochloric acid to a total volume of 400 mL is approximately 1.185 M.