I have a Cannondale H300. The tire says 28x 1 1/4 x 1 3/4 on it. I need a new tube but not sure the size to order. Any suggestions? Thank you!!

My money is on 700c x 32-47mm.

Look for the ISO size of the tire. It'll be on the sidewall somewhere. Probably embossed in the rubber in the tiniest print possible. It'll properly be nn - nnn but some brands use a bastardized version that harkens back to the French sizing system, even though it isn't anything like the French sizing. 28" can also be 700C (French sizing) or 622 ISO. If you find that on your sidewall, then the 2 digit number next to it is the width of the tire in millimeters.

Tubes are sold in a size that will fit a range of tire widths. So look for a 700C, 28" or 29" tube that also says the range of width's it's made for include the width stated on your tire. Note that 700C, 28" and 29" are all a tire with a 622mm bead seat diameter.

Tire sizing can be confusing if you only look at the inch sizing printed on them. That's more advertising related than a true size of your tire.

This might help you understand tire sizing...... https://www.sheldonbrown.com/tire-sizing.html

I wouldn't bet against MilhouseJ either. He is probably correct. Or very close on the width for it not to matter. But some brands use the phony tire size in big letters on the sides of their boxes. With them also, you have to look for the ISO sizes they fit.