Molecular weight of tap water should be 237 g

fluoride

also particle picked up in water lines, sediment etc.

Fluoride. Oh dear.....
Uncool

they add tons of bleach to our water.
it literally smells like someone poured bleach on the floor when you turn on the faucet.

past two days its been hard to breathe when you turn it on.

What temperature is your cup of water and what is the standard temp for weighing water?

It is below 62 degrees because my thermometer flashes 62 and then reads lo.

you need to zero out the plastic cup the water is held in

Measure at stp, standard temperature and pressure. That is, 20°C and whatever atmospheric pressure is at Sea level. 14psi? Something like that.

Even so, it won't account for that big a difference, 237g vs 280... Something's fucky. Have your water tested for heavy metal content, i.e. lead.

its mostly alkaline salts

I think you mean a cup of DISTILLED or PURE H2O should be 237, you know tap has many things in it and even if it were ultra clean tap it would be nothing like distilled

Tap water refers to a million different waters around the world, how could it be one value?

and tap water is never pure like distilled water even if its the cleanest in the world so you cane be referring to an idealism either.

water isn't always h2o like they tell us

It's all that dihydrogen monoxide! Kills thousands every year!

did you count the molecules of water in the cup? you have to make sure you have exactly 602000000000000000000000 molecules!!!

That is a huge difference. Try rainwater, river water, pond water - check your test methods. Rumor was they were poisoning us with heavy water but that would be $$$$$.