a, - Hiện tượng: Bột CuO từ đen chuyển đỏ đồng.

PT: \(CuO+CO\underrightarrow{t^o}Cu+CO_2\)

b, - Hiện tượng: Dd Br₂ nhạt màu dần rồi mất màu.

PT: \(C_2H_4+Br_2\rightarrow C_2H_4Br_2\)

c, - Hiện tượng: C₂H₂ cháy với ngọn lửa sáng, tỏa nhiều nhiệt.

PT: \(C_2H_2+\dfrac{5}{2}O_2\underrightarrow{t^o}2CO_2+H_2O\)

Đặt \(\left(a;2b;3c\right)=\left(x;y;z\right)\Rightarrow\left\{{}\begin{matrix}x;y;z\ge0\\x+y+z=1\end{matrix}\right.\)

\(M=xy-yz+zx\)

Ta có:

\(4M+1=4\left(xy-yz+zx\right)+\left(x+y+z\right)^2\)

\(=6xy+6zx+x^2+y^2+z^2-2yz\)

\(=\left(y-z\right)^2+x\left(6y+6z+x\right)\ge0\) (do \(x;y;z\ge0\))

\(\Rightarrow4M+1\ge0\)

\(\Rightarrow M\ge-\dfrac{1}{4}\)

\(M_{min}=-\dfrac{1}{4}\) khi \(\left\{{}\begin{matrix}x+y+z=1\\x=0\\y=z\end{matrix}\right.\) \(\Rightarrow\left(x;y;z\right)=\left(0;\dfrac{1}{2};\dfrac{1}{2}\right)\) hay \(\left(a;b;c\right)=\left(0;\dfrac{1}{4};\dfrac{1}{6}\right)\)

a, Đốt A thu CO₂ và H₂O nên A gồm C và H, có thể có O.

 \(n_{CO_2}=\dfrac{10,56}{44}=0,24\left(mol\right)=n_C\)

\(n_{H_2O}=\dfrac{4,32}{18}=0,24\left(mol\right)\Rightarrow n_H=0,24.2=0,48\left(mol\right)\)

⇒ m_C + m_H = 0,24.12 + 0,48.1 = 3,36 (g) < m_A

→ A gồm C, H và O.

⇒ m_O = 7,2 - 3,36 = 3,84 (g)

\(\Rightarrow n_O=\dfrac{3,84}{16}=0,24\left(mol\right)\)

Gọi CTPT của A là C_xH_yO_z.

⇒ x:y:z = 0,24:0,48:0,24 = 1:2:1

→ A có dạng (CH₂O)_n

Mà: M_A = 30.2 = 60 (g/mol)

\(\Rightarrow n=\dfrac{60}{12+2+16}=2\)

→ A là C₂H₄O₂.

b, A là axit hữu cơ.

→ A: CH₃COOH

TK:

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Teenagers play an extremely important role in current and future society for many reasons such as: First, they are the foundation of our country's future development, if they develop and work hard to cultivate their future. By gaining knowledge, they will contribute to the country becoming rich and strong. Second, they have a lot of creativity to invent modern products to help life. Finally, they have many ways to solve problems. Problems sometimes do not always follow the framework. Therefore, as teenagers, we need to try to cultivate knowledge and creativity. Try to become a little better than yesterday to build a more developed country and society in the future.