Chemistry SCH4U

Question

Instant hot packs work by crystallizing sodium acetate. the molar enthalpy of crystallization of sodium acetate is -56.7 kj/mol. How many grams of sodium acetate are needed to warm 125.0ml of water from 21 degree celsius to 35.04 degree celsius.

Answer 1

The proper equation comes from equating the energy gain by the water to the energy release of the hot pack.

m_wc_wΔT = -nΔH_cryst = -(m_SA/M_SA)ΔH_cryst

Solve for m:

m_SA = m_wc_wΔT(M_SA/(-ΔH_cryst))

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