Question 5 in the morning session of the 2010 case
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Should I choose C or D for this question? The unit mass of wood that I understand refers to wet wood, but it seems that all the answers given are based on dry wood? 5. Wood with a moisture content of 50 wt% on a wet basis is dried to a moisture content of 30 wt% on a wet basis. What value is closest to the amount of water evaporated per unit mass of wood (kg of water/kg of wood)? / m/ {, h; U. c1 P' u: F(A)0.43 (B) 0.78 (C)0.57 (D) 0.294
First, understand it directly as: how much water evaporates per unit mass of wood when it is dried from 50% moisture content to 30%? Doesn’t it seem to be on a dry basis from the wording? Please give me some advice! The exam questions are worth 2 points each, and they’re not difficult?
Assuming the wood weighs 1 kg, 1 x (1-50%) = X x (1-30%); thus X = 0.71 kg (after drying). The amount of water that evaporates is 1 - 0.71 = 0.29
If what is being sought is the amount of water evaporated per kilogram of wet wood, then that’s all there is to it. If what is being calculated is the amount of water that evaporates per kilogram of dry wood, then one more step is required: 0.29/0.5=0.58
8. The acid-base neutralization reaction takes place under standard conditions (25°C, 1 atm): ?5 f6 D1 G$ d+ \2NaOH(aq)+H2SO4(L)=Na2SO4(aq)+2H2O(L). H2 \; g/Xaq indicates that NaOH and Na2SO4 are in an infinitely dilute solution. + The standard formation enthalpy ΔHfθ and the standard integral heat in an infinitely dilute solution ΔHSθ are as follows: Compound ΔHfθ J/(g·mol) ΔHSθ J/(g·mol) H2O -427010 -42898 NaOH -42898 + W% p6 R( X) \4 ? H2SO4 -811860 -96255 Na2SO4 -1385400 -2344.6 H2O -286030 The standard reaction enthalpy J/(g·mol) for the above reaction is which of the following? (A) -497120 (B) -208128.6 (C) -936310 (D) 936310
Dry basis moisture content: X1 = w1/(1 – w1) = 0.5/(1 – 0.5) = 1; X2 = 0.3/(1 – 0.3) = 0.43. The amount of water that needs to be evaporated is W = G * (X1 – X2); thus, W/G = X1 – X2 = 1 – 0.43 = 0.57 KG of H2O per KG of wood. The correct answer is (C). If you can get questions like this wrong, think carefully about it!
It’s dry; there’s no need to indicate it as wet if that’s the case!
Thank you. I got it right the first time I tried, but when I tried again, I had this question: how should wetness be represented?
I have a question about question 8 on floor 5 – the result I got is different from the answer given
Answer to Question 8: B. Analysis of the key point: Standard reaction enthalpy = Products (standard formation enthalpies + standard integral enthalpies) – Reactants (standard formation enthalpies + standard integral enthalpies).
Question 5 is quite straightforward: simply apply the formula for wet air, and the amount of water removed per unit mass of dry wood is the difference between the two humidity levels, namely X1 – X2 = 1 – 3/7 = 0.57.