Calculate the amount of stock reagent(s) plus water.

0.050 mL of 2.5 mM Tris-HCL from 1.0 M stock.

Can you help me figure this out please?

the dilution formula
C1V1 = C2V2

(1.0 M) (V1) = (0.0025 M) (0.050 ml)
V1 = 0.000125 ml

you need to use 0.125 microlitres of 1.0 Molar, and dilute it to o.050 ml

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instead you can make up 10.00 ml of 2.5 mM in a volumetric flask
C1V1 = C2V2

(1.0 M) (V1) = (0.0025 M) (10.00 ml)
V1 = 0.025 ml

you need to use 25 microlitres of 1.0 Molar, and dilute it 10.00 ml in a volumetric flask....
the strength would be correct: 2.5 milliMolar (aka 0.0025 Molar)

then you wouls just micropipet out & use 0.050 ml at will