50g质量分数为36．5%的盐酸与30g大理石恰好完全反应（大理石中的杂质不参加反应）。
（1）计算反应生成二氧化碳气体的质量。
（2）计算该大理石中碳酸钙的质量分数。
解：设反应生成二氧化碳气体的质量为x，30g大理石中碳酸钙的质量为y。
CaCO3 + 2HCl = CaCl2+H2O+ CO2↑ ················ ·1分
100 2×36．5 44
y 50g×36．5% x
73︰44 = 18.25g︰x x=11g ···········1分
73︰100 = 18.25g︰y y=25g ············1分
大理石中碳酸钙的质量分数为： ×100%≈83.3% ···········1分
答：（略）
本题考查的就是根据化学方程式的计算，已知量为盐酸的溶质质量，50g×36．5%=18.25g，据此就可以求二氧化碳和碳酸钙的质量了，具体步骤见答案。
| 阅读理解. | ||
| ||
| his feet and a sign that read, "I am blind. Please help me." A creative publicist (广告员) named Tom was walking by the blind man and stopped to see that the man only had a few coins in his hat. He put a few of his own corns in the hat. Without asking for permission, took the sign, turned it around and wrote a new message. Then he put the sign by the feet of the blind man, and left. Later that afternoon the publicist returned to the blind man and noticed that his hat was almost full of bills and coins. The blind man recognized(识别)his footsteps and asked if it was he who had changed his sign. He also wanted to know what the man wrote on it. The publicist said, "I just wrote the message a little differently." He smiled and went on his way. The new sign read, "Spring has come, but I can't see anything." | ||
| 1. What's the blind man's name? | ||
| A. Tom. B. John. C. Tony. D. Sorry, I don't know. | ||
| 2. Who wrote a new message? | ||
| A. The blind man. B. The teacher. C. The publicist. D. The farmer. | ||
| 3. How did the blind man recognize the publicist? | ||
| A. By his sign. B. By his footsteps. C. By his voice. D. By his corns. | ||
| 4. When did the story happen? | ||
| A. In spring. B. In summer. C. In autumn. D. In winter. | ||
| 5. What can we learn from the passage? | ||
| A. Publicists are always friendly and helpful. B. We can get more money by changing a sign. C. The blind man is very stupid, so he has little money. D. The power (力量) of language is great. |
