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**Your exams are over and a new academic session has begun! So, Topper helps you gear up by answering ALL your queries, within 72 hours! Here are four questions asked by Topper readers from Class IX, and the answers by Topper maths and science experts. **

**Subject**:** ****Chemistry **

**Posted by**: **Iona**** Dias**

**A**. 1 mole of N_{2 }gas contains 2 x 6.023 x 10^{23} atoms of N because N_{2 }gas is a diatomic molecule. 1 molecule of N_{2 }gas is made up of 2 atoms of N

For 0.07 g of N_{2 }gas, number of moles =0.07/28=2.5 x 10^{-3}

If 1 mole of N_{2 }gas contains 2.5 x 10^{-3 }atoms of N

Therefore 2.5 x 10^{-3 mol} of N_{2 }gas contains 2.5 x 10^{-3 }x 2 x 6.023 x 10^{23} =3.011x 10^{21 }atoms of N.

****

**Subject**:** Physics**

** **

**Q**.** **The distance b/w Delhi and Agra is 200 m. A train travels the first 100 km at a speed of 50 km/hr.** **How fast must the train travel the next 100 km, so as to average 70 km/hr for the whole journey?

**Posted by**: **JSharma**

** **

**A**. The average speed = total distance / total time

The average speed required = 70km/hr

Total distance = 200 km

Speed of the train for the first 100 km = 50 km/hr

Time taken= 100/50 = 2 hours

Let the speed for the next 100 km be x km/hr

Time taken = 100/x

The total time taken = 2 +100 / x

Hence 70 = 200 / [ 2+100/x]

2+ 100/x = 20/7

x = 116.67 km/hr

Hence, the train must move with a speed of 116.67 km/hr.

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