At the same conditions of temperature and pressure, 5.0 dm^3 of a gas of molecular formula C_2 H_2, has a mass of 15 g, while 1.0 dm^3 of gas D has a mass of 1.4 g. What is the molar mass of gas D? Underscore means subscript but I can't get the equation thingy to work. So it's meant to be C(subscript 2)H(subscript 2)

Two metal blocks that have slightly different temperatures are placed next to one another. after five minutes, they both have lower but equal temperatures. according to the law of conservation of energy, what  most likelyhappened? energy was created inside the blocks.energy was destroyed inside the blocks.energy was absorbed into the blocks from outside the system.energy was transferred from the warmer block to the cooler block.