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A hydrogren sample gas collected over water at 25°C and 752 torr occupies 53.5 mL.Calculate its volume at STP condition.

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Combined gas law is used when P, T and V are given in the problem. Essentially it is a combined Charles's, Boyle's and Gay Lussac's Laws. It is expressed as:

P1V1/T1 = P2V2/T2

First we have to show the values of the given.

P1 = 752 torr

V1 = 53.5 mL

T1 = 25 + 273.15 = 298.15K

P2 = 760 torr (at STP)

T2 = 0 + 273.15 = 273.15 K (at STP)

V2 = ? (mL)


We can now solve for the value of V2.

P1V1/T1 = P2V2/T2

arranging the expression we can have:

V2 = (P1V1/T1) x (T2/P2)

Substitute the value and solve for V2.

V2 = (752 x 53.5/298.15) x (273.15/ 760)

V2 = 48.498 = 48.5 mL

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