In TDS experiment , the mass of the empty beaker was 25.683g , and the mass of the beaker after water evaporated 25.687 g , if the volume of the sample 50 ml , then the TDS of the water sample is ?

25.687g = mass beaker + sample

-25.683g = mass MT beaker
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0.004g = mass TDS
TDS = 0.004/50 mL. You convert it to whatever units you want.