What is the Biblical support for praying for the dead? In the question Absolution for a daughter an answer was posted that started by saying:


  The Rites of the Church do include prayers for the dead. It's not
  possible actually to absolve sins after death, but the dead are
  effectively helped by the prayers of the living.
  
  (emphasis added)


There is Biblical support that supports the idea of death sleep:

The dead do not know anything:


  For the living know that they will die; But the dead know nothing, And
  they have no more reward, For the memory of them is forgotten.
  
  Ecclesiastes 9:5


The dead cannot do anything:


  Whatever your hand finds to do, do it with all your might; for there
  is no activity or planning or knowledge or wisdom in Sheol where you
  are going.
  
  Ecclesiastes 9:10


The dead cannot praise God, or hope in His faithfulness:


  For the dead cannot praise you; they cannot raise their voices in
  praise. Those who go down to the grave can no longer hope in your
  faithfulness.
  
  Isaiah 38:18


There is no mention of God by the dead:


  For there is no mention of You in death; In Sheol who will give You
  thanks?
  
  Psalms 6:5


Jesus mentions that Lazarus, who was at the time dead, was simply sleeping:


  11 These things He said, and after that He said to them, “Our friend
  Lazarus sleeps, but I go that I may wake him up.”
  
  12 Then His disciples said, “Lord, if he sleeps he will get well.” 13
  However, Jesus spoke of his death, but they thought that He was
  speaking about taking rest in sleep.
  
  14 Then Jesus said to them plainly, “Lazarus is dead. 15 And I am glad
  for your sakes that I was not there, that you may believe.
  Nevertheless let us go to him.”
  
  John 11:11-15


Paul says that those who sleep, or died in Christ, simply wait for His return and their resurrection.


  13 But I do not want you to be ignorant, brethren, concerning those
  who have fallen asleep, lest you sorrow as others who have no hope. 14
  For if we believe that Jesus died and rose again, even so God will
  bring with Him those who sleep in Jesus.[b]
  
  15 For this we say to you by the word of the Lord, that we who are
  alive and remain until the coming of the Lord will by no means precede
  those who are asleep. 16 For the Lord Himself will descend from heaven
  with a shout, with the voice of an archangel, and with the trumpet of
  God. And the dead in Christ will rise first. 17 Then we who are alive
  and remain shall be caught up together with them in the clouds to meet
  the Lord in the air. And thus we shall always be with the Lord. 18
  Therefore comfort one another with these words.
  
  1 Thessalonians 4:13-18


In light of these texts, and many others not shown, what is the Biblical support which Catholics might adduce for praying for the dead or that our prayers help them in any way? And if so, does it contradict the texts presented? How would a Catholic convince a sola scriptura Protestant?
Catholics and Orthodox do not subscribe to the doctrine of Sola Scriptura, which is the doctrine that says that the only source of authoritative teaching is Sacred Scripture. Instead, we rely on Apostolic Succession to show that the doctrines we hold onto are those that the Apostles taught, by tracing the teachers and the teaching in an unbroken chain back to the Apostolic era. Regarding the specific issue of praying for the Dead, we have both the Sacramentary of Serapion and a Homily from St. John Chrysostom attesting to the antiquity of the practice already in the Fourth Century.
However, since the question asks for specifically Scriptural support for prayers for the Dead, the most important passage is the end of 2nd Maccabees 12:

39 And upon the day following, as the use had been, Judas and his company came to take up the bodies of them that were slain, and to bury them with their kinsmen in their fathers' graves.
40 Now under the coats of every one that was slain they found things consecrated to the idols of the Jamnites, which is forbidden the Jews by the law. Then every man saw that this was the cause wherefore they were slain.
41 All men therefore praising the Lord, the righteous Judge, who had opened the things that were hid,
42 Betook themselves unto prayer, and besought him that the sin committed might wholly be put out of remembrance. Besides, that noble Judas exhorted the people to keep themselves from sin, forsomuch as they saw before their eyes the things that came to pass for the sins of those that were slain.
43 And when he had made a gathering throughout the company to the sum of two thousand drachms of silver, he sent it to Jerusalem to offer a sin offering, doing therein very well and honestly, in that he was mindful of the resurrection:
44 For if he had not hoped that they that were slain should have risen again, it had been superfluous and vain to pray for the dead.
45 And also in that he perceived that there was great favour laid up for those that died godly, it was an holy and good thought. Whereupon he made a reconciliation for the dead, that they might be delivered from sin.

Here, part of Judas Maccabeus's army died fighting the Gorgias, military governor of Idumea. It was found that those who died had kept Jamnite idols, which is a sin. However, they had fought for Israel and to defend their homeland, so the people prayed that the Lord might forgive them this offense so that they might be raised when the Resurrection came. As verse 44 says, &quot;for if he had not hoped that they that were slain should have risen again, it had been superfluous and vain to pray for the dead.&quot; But because he hoped for the Resurrection, it is not vain to pray for them.
Of course, your argument is that prayer for the dead is vain because they are all asleep. In fact, the doctrine of Mortalism (sometimes known as &quot;soul sleep&quot;) is postulated exactly to justify the vanity of praying for the dead, or of intercessory prayer by the dead. But there are passages in Scripture where we see the dead specifically not sleeping. Particularly, in the passage of the Transfiguration (Mt 17:1–13; Mk 9:2–13; Lk 9:28–36) Moses and Elijah are standing together with Jesus and talking.

Mt 17:3 And, behold, there appeared unto them Moses and Elias talking with him.
Mk 9:4 And there appeared unto them Elias with Moses: and they were talking with Jesus.
Lk 9:30–31 And, behold, there talked with him two men, which were Moses and Elias: 31 Who appeared in glory, and spake of his decease which he should accomplish at Jerusalem.

Also, in the parable of the Rich Man and Lazarus (Lk 16:19–31), the deceased are shown awake and concerned about things on Earth:

23 And in hell he lift up his eyes, being in torments, and seeth Abraham afar off, and Lazarus in his bosom. 24 And he cried and said, Father Abraham, have mercy on me, and send Lazarus, that he may dip the tip of his finger in water, and cool my tongue; for I am tormented in this flame.

In Revelation, too, nowhere are the dead in Christ described as sleeping. Particularly, the twenty-four Elders in Revelation 4:4.10–11:

4 And round about the throne [were] four and twenty seats: and upon the seats I saw four and twenty elders sitting, clothed in white raiment; and they had on their heads crowns of gold.
10 The four and twenty elders fall down before him that sat on the throne, and worship him that liveth for ever and ever, and cast their crowns before the throne, saying,
11 Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created.

In the Psalms, a few references to the state of souls after death, for example:

Ps 16(17):9–11 Therefore my heart is glad, and my glory rejoiceth: my flesh also shall rest in hope. 10 For thou wilt not leave my soul in hell; neither wilt thou suffer thine Holy One to see corruption. 11 Thou wilt shew me the path of life: in thy presence [is] fulness of joy; at thy right hand [there are] pleasures for evermore.

In conclusion, there are many different positions on the nature of the soul and their destination after death in the Old Testament; and in the New Testament, the wide use of the verb κοιμάω &quot;to sleep&quot; to stand for θνῄσκω &quot;to die&quot; causes a misunderstanding (which unfortunately Luther was party to, which caused it to become a widely-held opinion among Protestants) that death is a literal sleep. This was not taught by the primitive Christian communities, as any review of the praxis of these communities will attest.