You have to convert the ppm to percent. 1 ppm is equivalent to 0.0001%. Therefore, 10 ppm formaldehyde is equivalent to 0.001% formaldehyde. By using C1V1=C2V2, where C1 is 40% formaldehyde, V1 is volume of 40% formaldehyde, C2 is 0.001% formaldehyde, find your V2 (volume of 0.001% formaldehyde).

V2=C1V1/C2. A log serial dilution will be of help to avoid obtaining a large V2.

Thank you Mr. Obi

Hello 40% is mean 400 000ppm

For making 10 ppm solution you need to perform dilution.

So 1ml +199ml in 200ml volumetric flask to have 2000 ppm

Then again 1ml from obtained solution +199ml in 200ml volumetric flask and you will receive 10ppm

Good luck!

Thank you Mr. Karen for answer.