题目

Mere is an astonishing and significant fact: Mental work alone can’t make us tired. It sounds absurd. But a few years ago, scientists tried to find out how long the human brain could labor without reaching a stage of fatigue (疲劳).To the amazement of these scientists, they discovered that blood passing through the brain, when it is active，shows no fatigue at all! If we took a drop of blood from a day laborer, we would find it full of fatigue toxins (毒素）and fatigue products. But if we took blood from the brain of an Albert Einstein, it would show no fatigue toxins at the en 测定铜合金中Cu的含量时，可以用硝酸溶解样品，也可用双氧水和盐酸溶解样品． (1) 用双氧水和盐酸溶解样品时，发生反应的化学方程式是________，此法与硝酸溶解样品相比较，哪种更好？________为什么？________． (2) 将0.500 g某铜合金样品溶解后，加热煮沸至无气体产生为止，冷却至室温，加入足量的已经酸化的KI溶液，发生如下反应：2Cu2+＋4I－2CuI＋I2．完全反应后，设法除去CuI，再用0.120 mol·L－1的Na2S2O3溶液滴定(I2＋2S2O2I－＋S4O)，消耗了35.5 mL　Na2S2O3溶液．则这种铜合金中Cu的质量分数是(实验时采取了适当措施使合金中的其他金属对测定无影响)________．