题目

Early in the twentieth century, two famous scientists developed different ideas about dreams. Austrian psychiatrist Sigmund Freud published a book called The Interpretation of Dreams in 1900. Freud believed people often dream about things they want but cannot have. These dreams are often linked to sex and aggression. For Freud, dreams were full of hidden meaning. He tried to understand dreams as a way to understand people and why they acted or thought in certain ways. Freud believed that every thought and every action started deep in our brains. He thought dreams could be an important way to 15．利用如图装置测定空气中氧气含量，实验步骤如下（装置气密性良好，部分操作已略去，因导管产生的误差忽略不计）：（1）打开止水夹a、b，向集气瓶中缓慢通入一定量的空气，通入集气瓶内空气的量应该满足的条件是白磷与空气接触，测得进入量筒中水的体积为V1．（2）关闭止水夹a、b，用强光照射引燃白磷，发生反应的化学方程式是4P+5O2$\frac{\underline{\;点燃\;}}{\;}$2P2O5．（3）白磷熄灭并冷却至室温，打开b，看到的现象是量筒中的倒吸入到集气瓶中，测得量筒内水的体积是V2．根据实验记录V1、V2，得出空气中氧气的体积分数的计算式是$\frac{{V}_{1}-{V}_{2}}{{V}_{1}}×100%$．