Hello,

It looks like you have lawn violet.  This weed can be difficult to kill and will likely need two applications (30 days apart) applied in the fall.  You will need a lawn herbicide with Triclopyr.  One example is:

Ortho Weed B Gon 32 oz. Chickweed, Clover and Oxalis Killer For Lawns Ready-To-Spray.

This comes with a hose-end sprayer attachment.  Make sure to spray the area farthest from your hose first, and work backwards, so you are not pulling the hose across the area that you just sprayed.

If you want to mix it yourself, and you have a sprayer, a better option is T-Zone because you can get a more precise rate.

Be sure and follow all label instructions and wear appropriate personal protective equipment (nitrile or rubber gloves, long sleeve shirt, pants and rubber shoes).  Also, do not apply in temperatures above 85 degrees and do not water for 24 hours after the application.

Finally, keep you and your pets off the areas you spray for 24 hours.

Good luck.